Electrode falloff detection

ABSTRACT

Systems for detecting contact between an electrode and a patient&#39;s skin using one or more contact detection schemes are provided. An example system can include an electrode assembly comprising at least one electrode configured to be disposed substantially proximate to the patient&#39;s skin and configured to at least one of sense an ECG signal of the patient and provide one or more therapeutic pulses to the patient, one or more sensors disposed on the electrode assembly and isolated from the electrode, the sensors configured to measure one or more properties to determine contact between the electrode and the patient&#39;s skin, and a controller configured to receive data representing the measured one or more properties and determine, based at least in part on the received data, whether the electrode is in contact with the patient&#39;s skin. The sensors can include temperature, impedance, capacitance, optical, and other similar sensors.

BACKGROUND

The present disclosure is directed to a wearable medical device, andmore specifically, to a wearable medical device configured to detect afalloff event associated with one or more electrodes.

There are a wide variety of electronic and mechanical devices formonitoring and treating patients' medical conditions. In some examples,depending on the underlying medical condition being monitored ortreated, medical devices such as cardiac monitors or defibrillators maybe surgically implanted or externally connected to the patient. In somecases, physicians may use medical devices alone or in combination withdrug therapies to treat conditions such as cardiac arrhythmias.

One of the deadliest cardiac arrhythmias is ventricular fibrillation,which occurs when normal, regular electrical impulses are replaced byirregular and rapid impulses, causing the heart muscle to stop normalcontractions and to begin to quiver. Normal blood flow ceases, and organdamage or death can result in minutes if normal heart contractions arenot restored. Because the victim has no perceptible warning of theimpending fibrillation, death often occurs before the necessary medicalassistance can arrive. Other cardiac arrhythmias can include excessivelyslow heart rates known as bradycardia or excessively fast heart ratesknown as tachycardia. Cardiac arrest can occur when a patient in whichvarious arrhythmias of the heart, such as ventricular fibrillation,ventricular tachycardia, pulseless electrical activity (PEA), andasystole (heart stops all electrical activity) result in the heartproviding insufficient levels of blood flow to the brain and other vitalorgans for the support of life.

Cardiac arrest and other cardiac health ailments are a major cause ofdeath worldwide. Various resuscitation efforts aim to maintain thebody's circulatory and respiratory systems during cardiac arrest in anattempt to save the life of the patient. The sooner these resuscitationefforts begin, the better the patient's chances of survival. Implantablecardioverter/defibrillators (ICDs) or external defibrillators (such asmanual defibrillators or automated external defibrillators (AEDs)) havesignificantly improved the ability to treat these otherwiselife-threatening conditions. Such devices operate by applying correctiveelectrical pulses directly to the patient's heart. Ventricularfibrillation or ventricular tachycardia can be treated by an implantedor external defibrillator, for example, by providing a therapeutic shockto the heart in an attempt to restore normal rhythm. To treat conditionssuch as bradycardia, an implanted or external pacing device can providepacing stimuli to the patient's heart until intrinsic cardiac electricalactivity returns.

Example external cardiac monitoring and/or treatment devices includecardiac monitors, the ZOLL LifeVest® wearable cardioverter defibrillatoravailable from ZOLL Medical Corporation, and the AED Plus also availablefrom ZOLL Medical Corporation.

SUMMARY

In certain implementations, a system can be provided for detectingcontact between an electrode and a patient's skin. The system caninclude an electrode assembly comprising at least one electrodeconfigured to be disposed substantially proximate to the patient's skinand configured to at least one of sense an ECG signal of the patient andprovide one or more therapeutic pulses to the patient; one or moresensors disposed on the electrode assembly and isolated from theelectrode, the one or more sensors configured to measure one or moreproperties to determine contact between the electrode and the patient'sskin; and a controller configured to receive data representing themeasured one or more properties and determine, based at least in part onthe received data, whether the electrode is in contact with thepatient's skin.

In certain implementations, the system can further include an alarmmodule operably configured to the controller and configured to output atleast one alarm if the controller determines that the electrode is notin contact with the patient's skin. In some examples, the at least onealarm can include at least one of an audio alarm, a visual alarm, atactile alarm, and combinations thereof.

In certain implementations, the system can further include a networkinterface operably connected to the controller and configured toestablish communication between the controller and a remote computingdevice such that, if the controller determines that the electrode is notin contact with the patient's skin, a notification is sent to the remotecomputing device indicating an electrode falloff event.

In certain implementations, the one or more sensors can be disposed onthe electrode. In some examples, the electrode can include an electrodeconfigured to sense at least one surface ECG signal, wherein theelectrode can include an impedance detection range selected from atleast one of 50Ω-200Ω, 200Ω-400Ω, 400Ω-10 kΩ, 10 kΩ-1 MΩ, and 1 MΩ-10MΩ.

In certain implementations, the one or more properties define a level ofcontact between the electrode and the patient's skin. In some examples,the controller can be further configured to compare the level of contactto a contact threshold level of contact to determine a falloff event.

In certain implementations, the one or more sensors are configured tomeasure an impedance level between the electrode and the patient's skin.In some examples, the controller can be further configured to model anelectrical circuit representative of an interface between the one ormore sensing locations and the patient's skin based at least upon themeasured impedance level, and determine whether the electrode is incontact with the patient's skin. In some examples, the modeledelectrical circuit can be configured to simulate an impedance levelbetween the electrode assembly and the patient's skin, the modeledelectrical circuit comprising at least a first cell configured tosimulate a stored energy level of the electrode, a first capacitive andresistive pair configured to simulate the electrode, a second capacitiveand resistance pair configured to simulate an electrolyte layerpositioned between the electrode and the patient's skin, a second cellconfigured to simulate an energy potential between the electrode and thepatient's skin, a second capacitive and resistance pair configured tosimulate an epidermis layer of the patient, and a resistance configuredto simulate a dermis layer of the patient.

In certain implementations, the one or more properties can include atleast one of temperature, capacitance, measured distance between theelectrode and the patient's body, and oxygen saturation of the patient'sblood.

In certain implementations, the one or more sensors can include acombination of multiple sensor types selected from at least atemperature sensor, a capacitive sensor, and an optical sensor. In someexamples, the multiple sensor types can be configured to operate inconcert to provide multiple measurements of the one or more propertiesdetermined by the position of the electrode in relation to the patient'sbody. In some examples, the controller can be further configured toreceive data representing the measured one or more properties from eachof the multiple sensor types to determine whether the electrode is incontact with the patient's body.

In certain implementations, a wearable medical system can be providedfor detecting contact between an electrode and a patient's skin. Thewearable medical system can include an externally wearable cardiacmonitoring device; an electrode configured to be coupled to theexternally wearable cardiac monitoring device and configured to bedisposed substantially proximate to the patient's skin to at least oneof sense an ECG signal of the patient and provide one or moretherapeutic pulses to the patient; at least one temperature sensordisposed on the electrode, the at least one temperature sensor tomeasure a value indicative of a temperature at an interface of theelectrode and the patient's skin; and a controller housed within theexternally wearable cardiac monitoring device, the controller configuredto receive data representing the measured value and determine, based atleast in part on the received data, whether the electrode is in contactwith the patient's skin.

In certain implementations of the wearable medical system, the at leastone temperature sensor can be disposed on a first surface of theelectrode positioned substantially proximate to the patient's skin, themedical system further comprising a second temperature sensor disposedon a second surface of the electrode and configured to be positionedaway from the patient's skin, the second temperature sensor configuredto measure ambient temperature.

In certain implementations of the wearable medical system, thecontroller can be configured to determine whether the electrode is incontact with the patient's body based on a determination of whether themeasured temperature has changed faster than a threshold rate of change.

In certain implementations of the wearable medical system, thecontroller can be configured to determine whether the electrode is incontact with the patient's body based on a determination of whether themeasured temperature has exceeded, for at least a threshold period oftime, a threshold of temperature change from an expected temperature.

In certain implementations of the wearable medical system, the wearablemedical system can further include an ambient temperature sensorconfigured to measure ambient temperature, wherein the controller isconfigured to receive data representing the ambient temperature.

In certain implementations of the wearable medical system, the wearablemedical system can further include an accelerometer to measure motionassociated with the sensing electrode, wherein the controller isconfigured to receive data representing the measured motion.

In certain implementations of the wearable medical system, the at leastone temperature sensor can include at least one of a thermocouple, athermistor, a resistance temperature detector, a pyrometer, and aninfrared temperature sensor.

In certain implementations of the wearable medical system, the at leastone temperature sensor can be thermally insulated from a surface of theelectrode. In some examples, the wearable medical system can furtherinclude an insulating material positioned between the at least onetemperature sensor and the surface of the electrode to thermallyinsulate the at least one temperature sensor.

In certain implementations, a medical system for detecting contactbetween an electrode and a patient's skin is provided. The systemincludes an externally wearable cardiac monitoring system; an electrodeconfigured to be coupled to the externally wearable cardiac monitoringdevice and configured to be disposed substantially proximate to thepatient's skin and configured to at least one of sense an ECG signal ofthe patient and provide one or more therapeutic pulses to the patient;at least one capacitive sensor disposed on the electrode and configuredto be positioned substantially proximate the patient's skin to measure acapacitance value between an interface of the electrode and thepatient's skin; and a controller housed within the externally wearablecardiac monitoring device, the controller configured to receive datarepresenting the measured capacitance value and determine, based atleast on the received data, whether the electrode is in contact with thepatient's skin. In some examples, the one or more sensing locationsconfigured to measure capacitance can include at least one of adielectric-based capacitive sensor, an electrostatic-based touch panel,and a resistive-based capacitance sensor.

In certain implementations, an alternative medical system for detectingcontact between an electrode and a patient's skin is provided. Thesystem includes an externally wearable cardiac monitoring system; anelectrode configured to be coupled to the externally wearable cardiacmonitoring device and configured to be disposed substantially proximateto the patient's skin and configured to at least one of sense an ECGsignal of the patient and provide one or more therapeutic pulses to thepatient; at least one optical sensor disposed on the electrode andconfigured to be positioned substantially proximate the patient's skinand to measure a value indicative of a distance between the electrodeand the patient's skin; and a controller housed within the externallywearable cardiac monitoring device, the controller configured to receivedata representing the measured value indicative of the distance betweenthe electrode and the patient's skin and determine, based at least onthe received data, whether the electrode is in contact with thepatient's skin. In some examples, the one or more sensing locationsconfigured to optically measure a distance between the electrode and thepatient's body can include at least one of a photoelectric sensor, aninfrared proximity sensor, and a pulse oximetry sensor. In someexamples, the pulse oximetry sensor can be configured to measure bloodoxygen saturation information for the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one example are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide an illustration anda further understanding of the various aspects and examples, and areincorporated in and constitute a part of this specification, but are notintended to limit the scope of the disclosure. The drawings, togetherwith the remainder of the specification, serve to explain principles andoperations of the described and claimed aspects and examples. In thefigures, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in every figure.

FIG. 1 depicts a wearable medical device, in accordance with an exampleof the present disclosure.

FIGS. 2A and 2B depict front and back views of a medical devicecontroller, in accordance with an example of the present disclosure.

FIG. 3 depicts a component schematic of a sample medical devicecontroller, in accordance with an example of the present disclosure.

FIG. 4 depicts an electrode including one or more temperature sensors,in accordance with an example of the present disclosure.

FIG. 5 depicts a sample processing circuit for an electrode having oneor more temperature sensors, in accordance with an example of thepresent disclosure.

FIG. 6 depicts a process flow for determining a falloff event in anelectrode having one or more temperature sensors, in accordance with anexample of the present disclosure.

FIG. 7 depicts a lumped circuit model for electric field sensing, inaccordance with an example of the present disclosure.

FIG. 8 depicts a circuit diagram representing the interaction between apatient's body part and an electric field sensor, in accordance with anexample of the present disclosure.

FIG. 9 depicts an electrode/skin model, in accordance with an example ofthe present disclosure.

FIG. 10 depicts a split electrode model, in accordance with an exampleof the present disclosure.

FIGS. 11A-F depict various examples of capacitance and/or impedancedetection circuits, in accordance with an example of the presentdisclosure.

FIG. 12 depicts an electrode including one or more capacitive sensors,in accordance with an example of the present disclosure.

FIG. 13 depicts an electrode including one or more touch sensors, inaccordance with an example of the present disclosure.

FIGS. 14A and 14B depict an electrode including various configurationsof electric-field proximity sensors, in accordance with an example ofthe present disclosure.

FIG. 15 depicts a process flow for determining a falloff event in anelectrode having one or more capacitive sensors, in accordance with anexample of the present disclosure.

FIG. 16 depicts an electrode including one or more optical sensors, inaccordance with an example of the present disclosure.

FIG. 17 depicts a sample circuit diagram for an optical sensor, inaccordance with an example of the present disclosure.

FIG. 18 depicts a process flow for determining a falloff event in anelectrode having one or more optical sensors, in accordance with anexample of the present disclosure.

FIG. 19 depicts a sample electrode-skin interface and an equivalentcircuit model, in accordance with an example of the present disclosure.

FIG. 20 depicts a process flow for determining a falloff event using aselectrode-skin interface circuit model, in accordance with an example ofthe present disclosure.

DETAILED DESCRIPTION

This disclosure relates to improvements in detecting that electrodesassociated with a wearable medical device are in contact with apatient's skin prior to, for example, delivering a therapy, or tomaintain a good connection for monitoring one or more physiologicalsignals of the patient.

Medical devices as described herein includes cardiac monitoring and/ortherapeutic devices as described in further detail below. For boththerapeutic and monitoring medical devices, maintaining one or moreelectrodes in their proper positions results in effective operation ofthe medical devices than when the one or more electrodes are notproperly positioned. Such electrodes are “dry” electrodes, e.g.,electrodes that are not attached to the patient's skin by an adhesive orwhere contact is mediated by conductive gel. Typically, a dry electrodeis placed directly on the skin and, as a result of the contact betweenthe electrode and the skin, perspiration can accumulate on the electrodesurface to provide an electrolytic connection with the patient. A dryelectrode can be constructed from a housing configured to hold variouscircuit components and a treated, anodized metal surface configured tocontact the patient's skin. For example, the treated, anodized metalsurface can be treated with a tantalum pentoxide coating.

Depending on the design, a dry electrode can be configured to have awide range of input impedances when in contact with a patient's skin.For example, the impedance as seen by the electrodes when in contactwith the patient's skin can be in excess of 400 ohms, typically in therange of tens to hundreds of mega ohms. In certain implementations, thedry electrodes can have an impedance range of 400Ω to 10 MΩ. In someexamples, a dry electrode can be a high impedance electrode having animpedance range of 10 MΩ to 100 MΩ, 100 MΩ to 1 GΩ, and 1 GΩ to 10 GΩ.It should be noted that these impedance ranges are provided by way ofexample only and can be configured based upon the design, manufacture,and intended use of the electrodes.

With such a design, having a high impedance range, by using a highfrequency signal (e.g., 1 kHz-100 kHz) with a micro-current signal(e.g., 10 μA), the impedance value measured by the electrode willincrease above, for example, 10 MΩ when the electrode loses contact withthe patient's skin.

Additionally, the dry electrode can be configured such that it operatesat a specific frequency range at a specific set of current densities.For example, the input signal to the dry electrode can have a frequencyranging from 0.5 Hz to 200 Hz, 1 Hz to 100 Hz, 10 Hz to 1 kHz, 10 kHz to100 kHz, and various other input frequency ranges. Similarly, theelectrode can be configured to operate at various current densities. Forexample, a dry electrode can have a contact surface area ofapproximately 10 cm² and an input current or approximately 10 μA. Assuch, the current density would be approximately 1 μA/cm². However, itshould be noted that this current density is shown by way of exampleonly and an electrode can be configured to operate at various otherfrequency densities. For example, the electrode can be configured tooperate at 0.1 μA/cm²-1.5 μA/cm², 1.5 μA/cm²-2.5 μA/cm², 2.5 μA/cm²-5μA/cm², 5 μA/cm²-10 μA/cm², and other similar frequency densities.

In certain implementations, when a dry electrode is properly positionedon a patient, a conductive surface of the electrode faces the patientand directly or indirectly (e.g., through an intervening conductive meshor other conductive surface) contacts the patient's skin. When properlypositioned, a therapy electrode (e.g., as included in a therapeuticmedical device) can apply a therapy, such as a defibrillation shock, tothe patient. Similarly, when properly positioned, a sensing electrode(e.g., as included in both therapeutic and monitoring medical devices)can measure various physiological signals including, but not limited to,ECG signals, heart sounds, tissue fluid levels, lung sounds, respirationsounds, patient movement, and other similar physiological signals of thepatient. Due to, for example, patient motion or improper installation bythe patient, health care provider, or other professional, an electrodecan sometimes be or become improperly positioned over time. As a result,an electrode's conductive surface may not make optimum contact with thepatient. A portion of an electrode can pull away from the patient'sskin, resulting in a reduced contact area with the patient's skin. Forexample, a sensing electrode can become detached from the garment or thegarment can become twisted or otherwise pulled away from the patient'sskin. Such conditions can result in a falloff event where the sensingelectrode partially (e.g., where a portion of a sensing electrode haslost contact with the patient's skin) or fully pulls away from thepatient's skin. In such a falloff event, the quality of informationbeing monitored by the sensing electrode can be reduced or, if thesensing electrode has lost all contact with the skin, can be reduced tozero. In a sensing electrode falloff event, a loss of contact with thepatient's skin can result in lower quality sensed electrocardiogramsignals. In a therapy electrode falloff event, a loss of contact withthe patient's skin can increase an electrode impedance and result in aless effective therapeutic shock.

As described herein, various techniques for detecting electrode falloffcan be used. For example, a temperature-based falloff detection schemecan be used. One or more temperature sensors can be integrated into theelectrodes. Output from the temperature sensors can be processed (e.g.,conditioned and filtered) prior to a medical device controller receivingthe temperature sensor outputs for further processing. Measured changesin an interface between the electrode and the patient's skin can befurther evaluated to determine if a falloff event has occurred, e.g.,has an electrode lost contact with the patient's skin.

Another technique can be a capacitance-based falloff detection scheme.One or more capacitance or touch sensors can be integrated into theelectrodes. The medical device controller can monitor the capacitancesensor outputs for the electrodes to detect an electrode falloff, e.g.,when some or all of the conductive portion of the sensing electrodeloses contact with the patient's skin. For example, the capacitancesensors can be configured to receive an input capacitance from apatient's body through contact between the patient's skin and thecapacitance sensor mounted in an electrode. The medical devicecontroller can receive a capacitive falloff signal including thecapacitance change information, process the information to determinethat the sensor has likely fallen off, and provide a notification to apatient that the sensor has likely fallen off.

An optical-based falloff detection scheme can be included in a wearablemedical device for detecting electrode falloff. In such a scheme, one ormore optical sensors can be integrated into the electrodes. The medicaldevice controller can monitor the optical sensor outputs to detect anelectrode falloff e.g., when some or all of the conductive portion ofthe sensing electrode loses contact with the patient's skin. Forexample, the optical sensors can be configured to both emit an opticalsignal and receive a reflected signal from the patient's skin. Byanalyzing properties of the received signal, and comparing the receivedsignal to the transmitted signal, a processing circuit can determine adistance measurement between the optical sensor and the patient's skin.The medical device controller, or a component of the medical devicecontroller such as the electrode falloff detector, can receive anoptical falloff signal including distance measurement and changeinformation, process the information to determine whether the sensor haslikely fallen off, and provide a notification to a patient that thesensor has likely fallen off.

In another example, an impedance-based falloff detection scheme can beincluded in a wearable medical device. In such a scheme, resistancemeasuring properties of an electrode can be used to measure changes inimpedance and/or capacitance between an electrode-skin interface, e.g.,the area of contact between the sensing and/or therapy electrode and apatient's skin. In such an example, the medical device controller canmonitor the outputs of each sensing electrode to determine changes inimpedance that could be indicative of a falloff event, e.g., when someor all of the conductive portion of the electrode loses contact with thepatient's skin.

Examples of measuring capacitance and impedance in a patient usingelectrodes can be found in U.S. patent application Ser. No. 14/843,843,titled “Impedance Spectroscopy for Defibrillator Applications,” filedSep. 2, 2015, the content of which is incorporated herein by referencein its entirety. Such techniques as described therein can beincorporated into the detection schemes as described in the presentdisclosure.

Various aspects and embodiments as described herein are directed to awearable monitoring and/or therapeutic device that can be fully orpartially worn by, for example, an ambulatory patient. A wearabletherapeutic device can include a garment with one or more pocketsconfigured to house at least one therapy electrode. The garment can alsoinclude one or more attachment points configured to releaseably hold atleast one sensing electrode. For example, the attachment point, and acorresponding sensing electrode, can use a hook-and-loop fastener toreleaseably attach the sensing electrode to the attachment point. Incertain implementations, the therapy electrodes and/or the sensingelectrodes can be attached directly to the patient using, for example, along-term adhesive.

Example Wearable Therapeutic Device

FIG. 1 illustrates an example medical device 100 that is external,ambulatory, and wearable by a patient 102, and configured to implementone or more configurations described herein. For example, the medicaldevice 100 can be a non-invasive medical device configured to be locatedsubstantially external to the patient. Such a medical device 100 can be,for example, an ambulatory medical device that is capable of anddesigned for moving with the patient as the patient goes about his orher daily routine. For example, the medical device 100 as describedherein can be bodily-attached to the patient such as the LifeVest®wearable cardioverter defibrillator available from ZOLL® MedicalCorporation. In one example scenario, such wearable defibrillators canbe worn nearly continuously or substantially continuously for two tothree months at a time. During the period of time in which they are wornby the patient, the wearable defibrillator can be configured tocontinuously or substantially continuously monitor the vital signs ofthe patient and, upon determination that treatment is required, can beconfigured to deliver one or more therapeutic electrical pulses to thepatient. For example, such therapeutic shocks can be pacing,defibrillation, or transcutaneous electrical nerve stimulation (TENS)pulses.

The medical device 100 can include one or more of the following: agarment 110, one or more sensing electrodes 112 (e.g., ECG electrodes),one or more therapy electrodes 114 a and 114 b (collectively referred toherein as therapy electrodes 114), a medical device controller 120, aconnection pod 130, a patient interface pod 140, a belt 150, or anycombination of these. In some examples, at least some of the componentsof the medical device 100 can be configured to be affixed to the garment110 (or in some examples, permanently integrated into the garment 110),which can be worn about the patient's torso.

The medical device controller 120 can be operatively coupled to thesensing electrodes 112, which can be affixed to the garment 110, e.g.,assembled into the garment 110 or removably attached to the garment,e.g., using hook and loop fasteners. In some implementations, thesensing electrodes 112 can be permanently integrated into the garment110. The medical device controller 120 can be operatively coupled to thetherapy electrodes 114. For example, the therapy electrodes 114 can alsobe assembled into the garment 110, or, in some implementations, thetherapy electrodes 114 can be permanently integrated into the garment110. Additionally, the therapy electrodes 114 can include one or moreconductive gel deployment devices such as the devices described hereinand, as other examples, devices described in U.S. Patent ApplicationPublication No. 2015/0005588 entitled “Therapeutic Device IncludingAcoustic Sensor,” the content of which is incorporate herein byreference.

Component configurations other than those shown in FIG. 1 are possible.For example, the sensing electrodes 112 can be configured to be attachedat various positions about the body of the patient 102. The sensingelectrodes 112 can be operatively coupled to the medical devicecontroller 120 through the connection pod 130. In some implementations,the sensing electrodes 112 can be adhesively attached to the patient102. In some implementations, the sensing electrodes 112 and at leastone of the therapy electrodes 114 can be included on a single integratedpatch and adhesively applied to the patient's body.

The sensing electrodes 112 can be configured to detect one or morecardiac signals. Examples of such signals include ECG signals and/orother sensed cardiac physiological signals from the patient. In certainimplementations, the sensing electrodes 112 can include additionalcomponents such as accelerometers, acoustic signal detecting devices,and other measuring devices for recording additional parameters. Forexample, the sensing electrodes 112 can also be configured to detectother types of patient physiological parameters and acoustic signals,such as tissue fluid levels, heart sounds, lung sounds, respirationsounds, patient movement, etc. Example sensing electrodes 112 include ametal electrode with an oxide coating such as tantalum pentoxideelectrodes, as described in, for example, U.S. Pat. No. 6,253,099entitled “Cardiac Monitoring Electrode Apparatus and Method,” thecontent of which is incorporate herein by reference.

In some examples, the therapy electrodes 114 can also be configured toinclude sensors configured to detect ECG signals as well as otherphysiological signals of the patient. The connection pod 130 can, insome examples, include a signal processor configured to amplify, filter,and digitize these cardiac signals prior to transmitting the cardiacsignals to the medical device controller 120. One or more of the therapyelectrodes 114 can be configured to deliver one or more therapeuticdefibrillating shocks to the body of the patient 102 when the medicaldevice 100 determines that such treatment is warranted based on thesignals detected by the sensing electrodes 112 and processed by themedical device controller 120. Example therapy electrodes 114 caninclude conductive metal electrodes such as stainless steel electrodesthat include, in certain implementations, one or more conductive geldeployment devices configured to deliver conductive gel to the metalelectrode prior to delivery of a therapeutic shock.

In some implementations, medical devices as described herein can beconfigured to switch between a therapeutic medical device and amonitoring medical device that is configured to only monitor a patient(e.g., not provide or perform any therapeutic functions). For example,therapeutic components such as the therapy electrodes 114 and associatedcircuitry can be optionally decoupled from (or coupled to) or switchedout of (or switched in to) the medical device. For example, a medicaldevice can have optional therapeutic elements (e.g., defibrillationand/or pacing electrodes, components, and associated circuitry) that areconfigured to operate in a therapeutic mode. The optional therapeuticelements can be physically decoupled from the medical device as a meansto convert the therapeutic medical device into a monitoring medicaldevice for a specific use (e.g., for operating in a monitoring-onlymode) or a patient. Alternatively, the optional therapeutic elements canbe deactivated (e.g., by means or a physical or a software switch),essentially rendering the therapeutic medical device as a monitoringmedical device for a specific physiologic purpose or a particularpatient. As an example of a software switch, an authorized person canaccess a protected user interface of the medical device and select apreconfigured option or perform some other user action via the userinterface to deactivate the therapeutic elements of the medical device.

Example Patient Monitoring Device

The wearable medical device can be a non-therapeutic patient monitoringdevice for an ambulatory patient, such as cardiac event monitoring (CEM)device or mobile cardiac telemetry (MCT) device. Such devices collectcardiac information, such as a patient electrocardiogram (ECG) data, andprovide the information to an external network or remote server on aperiodic basis. In some implementations, such devices can also recordECG data associated with a particular triggering event (e.g., anautomatically detected cardiac event or a patient reported symptom), andsend such data to a remote server for analysis. MCT devices can furthercomprise additional sensors for measuring non-ECG physiologicalparameters. Data from non-ECG sensors can be provided along with ECGrecordings for identified events.

Cardiac monitoring devices can be used for monitoring patient cardiacfunction for a predetermined interval (e.g., a number of days or weeks)to provide information about frequency and duration of cardiac eventsexperienced by a patient. Cardiac events that can be identified bypatient monitors can include, without limitation, one or more of atrialfibrillation, bradycardia, tachycardia, atrio-ventricular block,Lown-Ganong-Levine syndrome, atrial flutter, sino-atrial nodedysfunction, cerebral ischemia, syncope, atrial pause, and/or heartpalpitations. The collected information about identified cardiac eventscan be used, for example, to produce patient reports for time periods ofinterest.

A patient monitor (e.g., a cardiac monitor) can include a controller,similar to the controller 120 as shown in FIG. 1, though withoutoperably connected therapeutic components such as, for example, therapyelectrodes 114 as shown in FIG. 1. The patient monitor controller can becommunicatively coupled (e.g., wired or wirelessly coupled) to sensorsand/or electrodes appropriately positioned on patient to obtain signals(e.g., ECG data and/or heart sounds data from an acoustic sensor)indicative of cardiac activity. In some examples, the patient monitorcontroller can, in addition to cardiac monitoring, perform monitoring ofother relevant patient parameters, e.g., weight, glucose levels, bloodoxygen levels, and blood pressure. The patient monitor controller canalso comprise motion sensors to track patient movement. In someexamples, the patient monitor can be in the form of an application on ahandheld device, such as a smartphone, a personal digital assistant, ora tablet device.

The patient monitor can also include a physiological data processingcomponent for collecting and conditioning the physiological data priorto storing the data locally at computer-readable storage media on themonitor itself and/or transmitting the data to a remote server ordevice. In some examples, the patient monitor controller can furtherinclude a user interface module that allows the patient to manuallyenter information about a patient condition, and to initiate sendinginformation to the remote server.

Example Medical Device Controller

FIGS. 2A-2B illustrate an example of the medical device controller 120.The controller 120 can be powered by a rechargeable battery 212. Therechargeable battery 212 can be removable from a housing 206 of themedical device controller 120 to enable a patient and/or caregiver toswap a depleted (or near depleted) battery 212 for a charged battery.The controller 120 includes a user interface such as a touch screen 220that can provide information to the patient, caregiver, and/orbystanders. The patient and/or caregiver can interact with the touchscreen 220 to control the medical device 100. The controller 120 alsoincludes a speaker 204 for communicating information to the patient,caregiver, and/or the bystander. The controller 120 includes one or moreresponse buttons 210. In some examples, when the controller 120determines that the patient is experiencing cardiac arrhythmia, thespeaker 204 can issue an audible alarm to alert the patient andbystanders to the patient's medical condition. In some examples, thecontroller 120 can instruct the patient to press and hold one or both ofthe response buttons 210 to indicate that the patient is conscious,thereby instructing the medical device controller 120 to withhold thedelivery of therapeutic defibrillating shocks. If the patient does notrespond to an instruction from the controller 120, the medical device100 can determine that the patient is unconscious and proceed with thetreatment sequence, culminating in the delivery of one or moredefibrillating shocks to the body of the patient. The medical devicecontroller 120 can further include a port 202 to removably connectsensing devices (e.g., the sensing electrodes 112) and/or therapeuticdevices (e.g., the therapy electrodes 114) to the medical devicecontroller 120 (e.g., via the connection pod 130).

FIG. 3 shows a schematic of an example of the medical device controller120 of FIGS. 1, 2A, and 2B. The controller 120 includes at least oneprocessor 318, a sensor interface 312, an optional therapy deliveryinterface 302, data storage 304 (which can include patient data storage316), an optional network interface 306, a user interface 308 (e.g.,including the touch screen 220 shown in FIG. 2), and a battery 310. Thesensor interface 312 can be coupled to any one or combination of sensorsto receive information indicative of patient parameters. For example,the sensor interface 312 can be coupled to one or more sensing devicesincluding, for example, the sensing electrodes 112. The therapy deliveryinterface 302 (if included) can be coupled to one or more electrodesthat provide therapy to the patient including, for example, the therapyelectrodes 114. In some implementations, the therapy delivery interface302 can also be coupled to pacing electrodes and/or transcutaneouselectrical nerve stimulation (TENS) electrodes. The sensor interface 312and the therapy delivery interface 302 can implement a variety ofcoupling and communication techniques for facilitating the exchange ofdata between the sensors and/or therapy delivery devices and thecontroller 120.

In some examples, the network interface 306 can facilitate thecommunication of information between the controller 120 and one or moreother devices or entities over a communications network. For example,the network interface 306 can be configured to communicate with a remotecomputing device (e.g., a remote server 322) where a caregiver canaccess information related to the patient. In certain implementations,the network interface 306 can be configured to establish a wirelessconnection with the remote computing device. For example, the networkinterface can be configured to connect to a bridge device such as awireless router via a local area network such as a WiFi network toestablish communications with the remote computing device.

In some examples, the medical device controller 120 includes a cardiacevent detector 320 to monitor the cardiac activity of the patient andidentify cardiac events experienced by the patient based on receivedcardiac signals. In some examples, the cardiac event detector 320 canaccess patient templates (e.g., which can be stored in the data storage304 as patient data 316) that can assist the cardiac event detector 320in identifying cardiac events experienced by the particular patient.

In some implementations, the processor 318 includes one or moreprocessors that each can perform a series of instructions that result inmanipulated data and/or control the operation of the other components ofthe controller 120. An example processor architecture can be found inU.S. Patent Application Publication No. 2016/0103482 filed Dec. 18, 2015and entitled “System and Method for Conserving Power in a MedicalDevice,” the content of which is hereby incorporate by reference in itsentirety.

During operation of a medical device (e.g., a therapeutic medical deviceand/or a monitoring medical device), maintaining a good connectionbetween the electrodes and the patient's skin can result in higherquality monitoring signals received from sensing electrodes as well asproviding for higher quality therapy (e.g., defibrillation or pacingshocks) delivered to the patient. As such, various techniques formonitoring for electrode falloff can be utilized with a wearable medicaldevice. Referring again to FIG. 3, the medical device controller 120 canfurther include an electrode falloff detector 324 configured to monitorfor and identify electrode falloff during operation of the wearablemedical device. The medical device controller can cause one or moreelectrodes (e.g., the therapy electrodes 114) to provide anelectromagnetic signal (e.g., in the form of an 800 Hz square wave) ontothe patient. One or more electrodes (e.g., the sensing electrodes) candetect the signal as it passes through the patient's body. The electrodefalloff detector 324 can read the detected signal. In certainimplementations, the sensed signal is present only when the transmittingelectrodes are in contact with the patient. If an electrode is off ofthe patient, the value detected by the receiving electrodes and read bythe medical device controller will be approximately 0V for thatparticular transmitting electrode. Thus, in some implementations, themedical device controller can transmit the electromagnetic signal fromeach of the therapy electrodes in sequence. The electrode falloffdetector 324 can then monitor each of the sensing electrodes to confirmthat each sensing electrode has received the electromagnetic signal. If,for example, a particular sensing electrode does not receive anelectromagnetic signal that the other sensing electrodes are detecting,the medical device controller can determine that the sensing electrode(that did not receive the signal) has fallen off the patient. Similarly,if the medical device controller instructs a therapy electrode totransmit an electromagnetic signal, and no sensing electrodes receivethe electromagnetic signal, the electrode falloff detector 324 candetermine that the therapy electrode transmitting the signal has fallenoff the patient. As such, by stepping through the various electrodes,the medical device controller can continually monitor for electrodefalloff.

It should be noted that, in the above example, the therapy electrodeswere transmitting the electromagnetic signal, and the sensing electrodeswere receiving the electromagnetic signal, by way of example only. Incertain implementations, the sensing electrodes can be configured totransmit an electromagnetic signal. Similarly, the therapy electrodescan be configured to receive an electromagnetic signal.

Referring again to FIG. 3, the medical device controller 120 can furtherinclude an alarm module 326. It should be noted that the alarm module326 can be part of the medical device controller 120 or a separateelement of wearable therapeutic device 100. The alarm module 326 can beimplemented using hardware, software, or a combination of hardware andsoftware.

For instance, in some examples, the alarm module 326 can be implementedas a software component executed by the processor 318. Accordingly,instructions included in the alarm module 326 can cause the processor318 to configure one or more alarm profiles (e.g., stored within thedata storage 304) and notify intended recipients using the alarmprofiles. For example, the alarm profiles stored in data storage 304 caninclude a list of alarm conditions (e.g., excessive noise above apredetermined threshold, electrode falloff event, among others), type ofalarm or an alarm path (e.g., an audible or visual alert to the patient,a notification sent to a remote device or server, among others) andintended recipients of the alarm. In an example, the alarm module 326can be an application-specific integrated circuit (ASIC) or afield-programmable gate array (FPGA) circuit coupled to the processor318 and configured to manage alarm profiles in the data storage 304, anduse the alarms specified within the alarm profiles to notify theintended recipients. An example alarm profile is summarized in the tablebelow.

Alarm condition Alarm type or path Recipient Partial electrode falloffevent (e.g., Audible alert to patient (e.g., a Patient and/or otherpatient where only a portion of the gong alert), visual alert (e.g., ansurrogate (e.g., a caregiver) sensing electrode has lost contactanimation indicating the electrode Optionally, send notification withthe patient skin) that has fallen off), and/or tactile to a remoteentity (e.g., a alert indicating that the patient technical supportperson or should adjust garment and/or caregiver). electrode. The alertcan also indicate which electrode of multiple electrodes needsattention. Electrode falloff event Audible alert to patient (e.g., aPatient and/or other patient gong alert), visual alert (e.g., ansurrogate (e.g., a caregiver) animation indicating the electrode Sendnotification to a remote that has fallen off), and/or tactile entity(e.g., a technical alert indicating that the patient support person orcaregiver). should adjust garment and/or electrode. The alert can alsoindicate which electrode of multiple electrodes needs attention.Repeated partial falloff or Upon exceeding a predetermined Uponexceeding a complete falloff events number of falloff eventpredetermined number of occurrences (e.g., 5 events within falloff eventoccurrences 2 hours), an audible or visual within a period of time(e.g., 5 notification can be provided to the events within 2 hours),patient to contact a technical optionally, a notification can supportperson for further be sent to a predesignated examination of the device.remote entity (e.g., technical support or caregiver). Multiple partialfalloff or Audible alert to patient (e.g., a Patient and/or otherpatient complete falloff events occurring gong alert), visual alert(e.g., an surrogate (e.g., a caregiver) substantially simultaneously(e.g., animation indicating the electrode Send notification to a remotetwo or more electrodes afflicted that has fallen off), and/or tactileentity (e.g., a technical by partial falloff or complete alertindicating that the patient support person or caregiver). falloff eventsat substantially the should adjust garment and/or same time) electrodeand/or contact technical support or a caregiver for assistance.

In certain implementations, the alarm module 326 can be configured toprovide various alerts such as audio alerts, visual alerts, tactilealerts, and a combination of two or more alerts. For example, the alarmmodule can be configured to provide alerts via speaker 204 andtouchscreen 220 as described above in FIG. 2, as well as tactile alarmsvia a tactile feedback device. In some examples, alarm module 326 can beconfigured to provide a notification that the sensing electrodes 112 ortherapy electrodes 114 are improperly positioned or have fallen off, asdetermined by, for example, the electrode falloff detector 324. Thealarm module 326 can also be configured to provide a notification thatsensing electrodes 112 or therapy electrodes 114 are properlypositioned. For example, the electrode falloff detector can beconfigured to provide a signal if no falloff events have been detected.In response to this signal, the alarm module can provide an indication(e.g., via the touchscreen 220) that the electrodes are properlypositioned.

In the above example, the electrode falloff detector 324 can continuallymonitor the various electrodes to detect a falloff condition. Thefalloff detection process can be sequential in nature and includesstepping through each of the electrodes in turn to detect a falloffcondition. The electrode falloff detection process can also include ascheme where all electrodes can be monitored at essentially the sametime (e.g., as described below, various components and devices fordetecting falloff can be provided locally at the individual electrodes,independent of the other electrodes to facilitate such an electrodefalloff detection process).

It should be noted that the therapy electrodes 114 and the therapydelivery interface 302 as shown in FIG. 3 are optional components andare included to fully illustrate that the medical device controller canbe designed and/or configured to function as a controller for both atherapeutic medical device as well as a monitoring medical device. Forexample, if the medical device controller 120 is designed to be used ina monitoring medical device, the therapy electrodes and the therapydelivery interface can be included as optional components. In certainimplementations, a monitoring medical device can be converted to atherapeutic medical device by either adding (e.g., in a modularcomponent) the therapy delivery interface and associated therapyelectrodes, or by activating an existing therapy delivery interface,thereby converting a monitoring medical device to a therapeutic medicaldevice. Conversely, the opposite can be true where a therapeutic medicaldevice can be converted to a monitoring medical device can removing ordeactivating the therapy delivery interface and associated therapyelectrodes.

Temperature-Based Falloff Event Detection

In an example, as illustrated in FIGS. 4-6, a temperature-based falloffdetection scheme can be included in a wearable medical device. In such ascheme, one or more temperature sensors can be integrated into theelectrodes. A processing circuit can also be implemented into theelectrodes such that output from the temperature sensors can beprocessed (e.g., conditioned and filtered) prior to the medical devicecontroller receiving the temperature sensor outputs for furtherprocessing. In such an example, the medical device controller canmonitor the temperature sensor outputs for all the electrodessimultaneously (or, depending upon processing capabilities,substantially simultaneously) to detect an electrode falloff, e.g., whensome or all of the conductive portion of the sensing electrode losescontact with the patient's skin. For example, the temperature sensor mayprovide an output signal for a specific electrode indicating a large andsudden temperature change (e.g., a 3-15° F. drop in temperature in 10-50ms) in an interface between the electrode and the patient's skin. Thetemperature and corresponding time interval range provided above isintended as an illustration. Other temperature and time interval valuescan be used. Further, the amount of temperature change and correspondingtime interval may be established by configurable parameters duringinitial device set up. The medical device controller can receive thesignal including the temperature change information, process theinformation to determine that the sensor has likely fallen off, andprovide a notification to a patient. For example, the notification maybe provided to the patient as an audible, tactile, or visual alarm. Forexample, the controller can cause the medical device to display amessage on a display device such as touch screen 220. The medical devicecontroller can then instruct the patient to check the specific electrodeand verify that the electrode is making proper connection with thepatient's skin. The following discussion of FIGS. 4-6 providesadditional details related to electrode falloff detection usingtemperature sensors.

FIG. 4 illustrates a sample electrode 400 that uses a temperature-basedfalloff detection process. It should be noted that, as shown in FIG. 4,the electrode 400 can be an electrode having at least one sideconfigured to be placed against a patient's skin. For example, theelectrode 400 can be either a sensing electrode or a therapy electrodeas described above. The electrode 400 can be electrically coupled to awire 405 configured to carry signals such as physiological signals(e.g., ECG signals and/or heart or lung sound signals) measured by theelectrode 400 to another device such as connection pod 130 or controller120 as described above in reference to FIG. 1. For explanatory purposesonly, wire 405 as described herein can be configured to bi-directionallycarry signals between the electrode 400 and a medical device controller.

The electrode 400 can include multiple temperature sensors 410A, 410Band 410C disposed on the electrode 400. Although three sensors areshown, any number of sensors may be used. Further, these sensors areshown as being placed along a diagonal of the electrode. Yet variationsin locations of the sensors are possible. Further, the electrode itselfmay assume various shapes and as such the locations of the sensors maychange accordingly. In certain implementations, each of temperaturesensors 410A, 410B and 410C can be a thermocouple. A thermocouple is anelectrical device typically including two different conductors arrangedto form electrical junctions at differing temperatures. A thermocouplecan be configured to produce a temperature-dependent voltage as a resultof a thermocouple effect resulting between physical interactions betweenthe two conductors as a result of temperature changes. This voltage canbe measured and interpreted by an appropriate processing circuit tomeasure current temperature and temperature change over time.Thermocouples can be designed to be self-powered without any externalexcitation and, as such, can be packaged in a relatively small enclosurecompared to other methods of temperature measurement such as athermistor. However, it should be noted that additional temperaturesensors such as thermistors, resistance temperature detectors,pyrometers, infrared temperature sensors, and other thermometers andtemperature sensors can be used.

In certain implementations, depending upon the material used to make theconducting portion of the electrode 400, the electrode 400 can have alarge thermal mass. In such an example, when the electrode loses contactwith a patient's skin (i.e., there is a falloff event), the temperatureof the electrode 400 can gradually decrease rather than drop quickly. Assuch, one or more of the temperature sensors 410A, 410B, 410C can beinsulated from the material used to make the conducting portion ofelectrode 400. For example, as shown in FIG. 4, each temperature sensor410A, 410B, 410C can be positioned such that it is insulated byinsulating material 412A, 412B and 412C respectively. As shown in FIG.4, each of insulating material 412A, 412B, 412C can be similarly shapedto the temperature sensors 410A, 410B, 410C. However, it should be notedthat this shape is shown by way of example only, and additional shapescan be used for the insulating material 412A, 412B, 412C.

Based upon the size and design of electrode 400, various materials canbe used for the insulating material 412A, 412B, 412C. For example, athermoplastic foam such as polystyrene can be used for the insulatingmaterial 412A, 412B, 412C. Additional types of insulations such asaerogel, polyurethane, thermoset plastics, cellulose materials, andother similar insulations can be used for insulating material 412A,412B, 412C depending on the size, shape, and design of electrode 400.

Referring again to FIG. 4, each temperature sensor 410A, 410B, 410C canbe operably connected to a processing circuit 420. In certainimplementations, temperature sensor 410A can be electrically coupled tothe processing circuit 420 by connection 415A (e.g., a copper wireconfigured to transmit a signal produced by the temperature sensor 410Ato the processing circuit 420), temperature sensor 410B can beelectrically coupled to the processing circuit 420 by connection 415B,and temperature sensor 410C can be electrically coupled to theprocessing circuit 420 by connection 415C.

To continue the above example, each of temperature sensors 410A, 410B,410C can be implemented as a thermocouple. In such an example,connections 415A, 415B, 415C can be copper (or another similarlyconductive material) wires configured to carry an electrical signalgenerated by one or more of the thermocouples to the processing circuit420. Processing circuit 420 can then further process the electricalsignal generated by one or more of the thermocouples and transmit theprocessed signal through wire 405 to a medical device controller such ascontroller 120 as described above.

Depending upon the implementation of the electrode 400, the processingcircuit 420 can be implemented in various manners. For example, theprocessing circuit 420 can be a standalone processing device configuredto receive an analog signal from the temperature sensors 410A, 410B,410C, condition and filter the signal to minimize noise and interferenceeffects, and convert the analog signals to digital signals fortransmission to a controller over the wire 405. In certainimplementations, the processing circuit 420 can be implemented as adirect conversion analog-digital converter configured to generate adigital code for a specific voltage range received from the temperaturesensors 410A, 410B, 410C. The processing circuit 420 can be configuredto transmit the digital code over wire 405 to the medical devicecontroller for further processing.

In a specific example, T-type thermocouple can be used for temperaturesensors 410A, 410B, 410C. A T-type thermocouple includes a titanium leadand a copper/constantan lead. The two leads are joined at a junction.Based upon the material differences, as temperatures change at thejunction, a voltage produced by the junction changes as well. Forexample, for a T-type thermocouple, at 98° F. (approximately 37° C.), anoutput voltage of approximately 1.5 mV can be output by thethermocouple. At 90° F., the output voltage drops to approximately 1.28mV. By measuring the changes in output voltage from the thermocouples,the medical device controller (e.g., via the electrode falloff detector324) can monitor for an electrode falloff event. The thermocouples maybe selected based on its dynamic response to changes in sensedtemperatures. Further, the temperatures described above as forillustration only. Other temperatures and corresponding voltages arepossible.

As illustrated in FIG. 5, the processing circuit 420 can also includevarious circuit components for conditioning and filtering the signalsreceived from the temperature sensors 410A, 410B, 410C prior totransmitting the signals to the medical device controller. For example,as shown in FIG. 5, a signal conditioner 505 can be configured toreceive one or more signals from the temperature sensors, e.g.,temperature sensors 410A, 410B, 410C. The signal conditional 505 can beconfigured to convert the received signal into a format for transmissionto and reading by, for example, the medical device controller. Incertain implementations, the signal conditioner 505 can be implementedas an analog-to-digital converter configured to convert the analogsignals received from the temperature sensors to a signal formatted forprocessing by the medical device controller. In some examples, theoutput of the signal conditioner 505 can be passed to a buffer 510. Thebuffer 510 can be configured to function as a voltage amplifier thatfunctions to convert the output of the signal condition to a signaltuned specifically for processing by the medical device controller. Forexample, the buffer 510 can be implemented as a voltage buffer amplifierconfigured to transfer the conditioned signals received from the signalconditioner 505 (received from, for example, a low impedance source suchas temperature sensors 410A, 410B, 410C) to a high impendence levelcircuit such as the medical device controller. In some examples, thebuffer 510 can be implemented as a unity gain buffer where the outputsignal of the buffer 510 has the same or essentially the same impedanceas the input signal.

The output of the buffer 510 can be transferred to a filter 515 forfiltering of the signal prior to transmission to the medical devicecontroller. For example, the filter 515 can be a lowpass filterconfigured to preferentially attenuate frequencies below a cutofffrequency (e.g., 1 Hz) of the output of the buffer 510 for furtherprocessing by the medical device controller. In such an example, thefilter 515 can be configured to smooth the temperature data obtained bythe temperature sensors prior to transmitting the data to the medicaldevice controller. In other implementations, the filter can be ahighpass filter, a notch filter, or another similar filter familiar tothose skilled in the art.

For example, the medical device controller can instruct the processingcircuit (e.g., processing circuit 420) to sample the temperatureinformation at a cutoff frequency of approximately 1 kHz (i.e., onetemperature sample per millisecond). The output of the temperaturesensors can be a voltage signal that changes in response to a measuredvoltage change by the sensors, but does not have an associated timingparameter. As such, the filter 515 can be configured to attenuate thesensor output signal to an appropriate 1 kHz signal for furtherprocessing by the medical device controller. However, it should be notedthat a cutoff frequency of 1 kHz is provided by way of example only.Depending upon the operational parameters of the filter 515, and theprocessing capabilities of the medical device controller, the cutofffrequency can be set to various other values. For example, the medicaldevice controller can set the cutoff frequency between 100 Hz and 2.5kHz. Similarly, the medical device controller can set the cutofffrequency between 1 kHz and 100 kHz.

Referring again to FIG. 5, after the filter 515 attenuates the signal,the signal can be transmitted by the processing circuit to the medicaldevice controller for further analysis to determine whether there hasbeen any temperature change at the electrode that could indicateelectrode falloff. For example, the processing circuit 420 can beconfigured to output the signal on a certain wire or channel in wire 405for monitoring and analysis by the medical device controller. As notedabove, in certain implementations an electrode falloff detectorcomponent such as electrode falloff detector 324 (as shown in FIG. 3 anddescribed above) can be configured to monitor the output of theprocessing circuit 420 for any signal changes that could indicate one ormore electrodes has fallen off the patient. A more detailed descriptionof the functionality of an electrode falloff component is provided inthe following description of FIG. 6.

It should be noted that the components as shown in FIG. 5 are providedby way of example only and, based upon the design of, for example,electrode 400 and processing circuit 420, one or more components can beremoved from the design and one or more additional components can beadded. For example, the processing circuit 420 can be implemented as asingle integrated circuit configured to receive the output from thetemperature sensors 410A, 410B, 410C, filter and attenuate the output,convert the output to a digital signal, and forward the digital signalfor additional processing at, for example, the electrode falloffdetector 324.

Additionally, one or more of the temperature sensors can be in thermalcontact with the electrode 400, and the electrode falloff detection canbe enhanced by measuring a temperature difference between a thermallyinsulated sensor (e.g., 410A, 410B, 410C as shown in FIG. 4) and atleast one sensor that has a higher thermal conductivity with theelectrode 400 as compared to the insulated sensors. The higher degree ofthermal conductivity can be achieved by closer physical coupling betweena sensor and the electrode 400 such as a machined groove in theelectrode 400 in which the sensor is placed, or alternatively with athermally conductive adhesive such as epoxy to adhere the sensor to theelectrode. Because of the larger thermal mass of the electrode 400, thetemperature readings of the thermally insulated sensors will equilibrateto ambient temperature with significantly shorter time constants thanthe temperature sensors with better thermal conductivity with theelectrode. For example, in certain implementations, if the temperaturedifference exceeds 1 degree Celsius for more than 30 seconds, theelectrode is determined to have fallen off.

FIG. 6 illustrates a sample process for monitoring one or moreelectrodes for a measured temperature change at the one or moreelectrodes that could indicate an electrode falloff As noted above, acomponent such as an electrode falloff detector integrated in themedical device controller can be configured to monitor signals relatedto the various electrodes and determine whether one of the electrodeshas fallen off the patient. However, it should be noted that this is forexemplary purposes only. Another component such as a multi-functionprocessing device integrated into the medical device controller can beconfigured to monitor the electrodes for falloff events. As such, thefollowing description including an electrode falloff detector isintended to provide a description of the process used to determine afalloff event rather than define what specific hardware componentperforms the process.

Referring to FIG. 6, the electrode falloff detector can be configured toreceive 605 the temperature falloff signal from, for example, one ormore of the electrodes. In certain implementations, each electrode canbe operably connected to a node such as connection pod 130 as shown inFIG. 1. The node can include processing circuitry configured toconcatenate, multiplex, or otherwise combine the temperature falloffsignals from each of the electrodes into a single combined temperaturefalloff signal for transmission to the medical device controller. Insuch an example, the electrode falloff device can be configured toreceive 605 the combined temperature falloff signal, divide the combinedtemperature falloff signal into individual components related to theindividual electrodes, and process the individual components todetermine whether one or more falloff events have occurred. Thus, inthis example, the remainder of the process as shown in FIG. 6 can berepeated for each individual electrode that is associated with thecombined temperature falloff signal.

In some implementations, each electrode can have a direct connection tothe medical device controller. For example, the medical devicecontroller can include one or more external connectors into which one ormore electrodes can be directly connected. In such an example, theelectrode falloff detector can be configured to receive 605 thetemperature falloff signals directly from the individual electrodes.Thus, in this example, the entirety of the process as shown in FIG. 6can be repeated for each electrode operably connected to the electrodefalloff detector.

Referring again to FIG. 6, the electrode falloff detector can be furtherconfigured to determine 610 whether there has been a measurabletemperature change at an electrode as indicate by the electrodestemperature falloff signal. If the electrode falloff detector determines610 that there has not been a temperature change, the electrode falloffdetector can receive 605 an updated temperature falloff signal.

Conversely, if the electrode falloff detector does determine 610 thatthere has been a temperature change, the electrode falloff detector canfurther determine 615 whether the temperature change was a rapidtemperature change. Depending upon the programming and implementation ofthe electrode falloff detector, a particular time range and associatedchange in temperature can be used to label a temperature change asrapid. In certain implementations, if a temperature change of more than10 degrees occurs in less than 1 second, the temperature change can belabeled as rapid. In some example, if the temperature changes more than5 degrees in less than 0.5 seconds, the temperature change can belabeled as rapid. For example, the electrode falloff detector can beconfigured to label a temperature change of 11 degrees that occurred inapproximately 0.75 seconds as a rapid temperature change. The valuesdiscussed herein are for illustration only. Other temperature values andassociated time values are possible. The electrode falloff detector canbe configured to determine that a rapid change in temperature in aparticular period of time is indicative of a falloff event. If such adetermination is made, the electrode falloff detector can provide 625 anotification of a falloff event. Such a notification can be aninstruction for an appropriate component of the medical devicecontroller to issue an alarm of other similar notification to a patientor caregiver that one or more electrodes have fallen off. For example,the medical device controller can display a visual notification on auser interface such as touchscreen 220 (as described above in referenceto FIG. 2) to check a specific electrode for a potential falloff event.Depending upon the capabilities and the programming of the medicaldevice controller, the visual notification can be accompanied by anaudio alarm or some form of tactile feedback.

If the electrode falloff detector determines 615 that the temperaturechange was not a rapid change, the electrode falloff detector can befurther configured to determine 620 whether the temperature change waswithin an allowed threshold. Based upon various measured patientparameters and/or operating parameters of the medical device controllerand the electrode falloff detector, a certain threshold of temperaturechange can be determined as acceptable. In certain implementations, themedical device controller can include a baseline temperature for thepatient. The acceptable threshold can be set as plus or minus a certainnumber of degrees from the baseline temperature. For example, theacceptable threshold can be set as plus or minus 2.5° F. for aparticular patient. If the electrode falloff detector determines 620that a temperature change at an electrode falls within the acceptablethreshold, the electrode falloff detector can receive 605 an updatedtemperature falloff signal and repeat the process as shown in FIG. 6.Conversely, if the electrode falloff detector determines 620 that thetemperature change is not within an acceptable threshold, the electrodefalloff detector can provide 625 a notification of a potential falloffevent.

In certain implementations, the electrode falloff detector can provide625 an alarm to the patient indicating a potential falloff event. Forexample, the alarm can include a visual alarm, an audio alarm, a tactilealarm, a combination of alarms (e.g., alarms that are in a predefinedsequence or that overlap, such as, first, initiating a tactile alert,second, initiating an audible alert, and third, initiating a visualalert on the display, or another similar alarm configured to provide anindication or notification of the potential falloff event to the patientwearing the medical device. In certain implementations, an alarm manager(e.g., alarm module 326 as described above) can be configured to outputone or more alarms in response to a specific event occurring. Forexample, if a treatable cardiac event is detected, the alarm manager canbe configured to cause a high volume audible alarm to occur. In someexamples, a high volume audible alarm can be about 80 db as measured 1meter from the output device (e.g., a speaker or audio resonator). Inthe event of an electrode falloff detection, the alarm manager can beconfigured to output a lower volume alarm. For example, the alarmmanager can be configured to output an alarm about 6-12 db lower thanthe high volume alarm (e.g., an alarm ranging from 68-74 db). In someexamples, the alarm manager can be configured to output a visual alarm.For example, the alarm manager can flash a message or notification onthe medical device's screen (e.g., touchscreen 220 as described above)or another similar visual output device such as one or more LED outputs.In certain implementations, the alarm manager can be configured toprovide a tactile alarm as a standalone alarm or in combination with oneor more of the audio and visual alarms.

In addition to providing the patient notification of the potentialelectrode falloff, the electrode falloff detector can further provide625 a notification to a remote server or monitoring service of thepotential falloff. For example, the wearable medical device can beoperably connected to a remote server (e.g., remote server 322 asdescribed above) and can be configured to regularly transmit dataindicative of a patient's cardiac activity as well as any detectedevents that occur while the patient is wearing the medical device. Upondetection of a potential electrode falloff, the electrode falloffdetector can provide 625 a notification such as a time/date stamp and anassociated flag indicative of the potential falloff event. Upon reviewof the patient's information (e.g., by a technician or a patient'sphysician) collected by the remote server, the potential falloff eventcan be reviewed as well. In certain implementations, a high amount offalloff events (e.g., more than 5 every 2 hours) can be indicative thatthe patient needs to have their wearable medical device adjusted orreplaced.

In an example of the process as shown in FIG. 6, the electrode falloffdetector can determine that, over a series of signals representing 30seconds in time, a patient's temperature at a specific electrode hasdropped 0.75° F. As this temperature drop falls within a 2.5° F.threshold, the electrode falloff detector can determine that there hasnot been a falloff event. To provide higher accuracy of whether afalloff event has occurred, the electrode falloff detector can comparemeasurements from multiple electrodes. For example, if a smalltemperature drop (e.g., 0.5° F.) is consistently measured acrossmultiple electrodes, the electrode falloff detector can label thetemperature change as a change in patient's body temperature rather thana falloff event. Such labels can be recorded in memory as flags,temperature events, or other similar events for review later by, forexample, a technician or other person (such as a physician) reviewingpatient-related information recorded by the wearable medical device.

It should be noted that the thresholds used for temperature-basedfalloff detection can be determined individually for each patient usingthe wearable medical device. For example, a patient can participate in abaselining process including measuring the patient's temperature during,for example, a garment fitting for the wearable medical device when thepatient is first subscribed the device. Additionally, the patient can beinstructed to perform a physical activity such as a six-minute walk testto measure how the patient's body temperature changes during physicalactivity. In such an example, the thresholds can be dynamicallyalterable for a patient depending upon whether the patient is engaged inphysical activity. Additional information such as accelerometerinformation can also be measured during the physical activity todetermine what measurable parameters and characteristics as associatedwith the patient during physical activity.

Additionally, an electrode can include a temperature sensor formeasuring an ambient temperature around the electrode. For example, asshown in FIG. 4, temperature sensors 410A and 410C can be positionedsuch that they contact the patient's skin. Temperature sensor 410B canbe positioned on an electrode surface away from the patient's skin, andcan be configured to measure an ambient temperature around theelectrode. The ambient temperature information can be used to determinehow accurate and reliable a temperature-based falloff scheme is for agiven environment and situation. For example, if a patient is outside ona summer day and the ambient air temperature is around 95° F., themedical device controller can determine that the temperature-basedfalloff scheme is not likely to produce reliable information as afalloff event could result in a 3° F. change. Such a change could bewithin the acceptable threshold and, as such, not register properly as afalloff event. In such an example, the temperature-based falloffdetection scheme can be used in concert with another detection scheme asdescribed herein.

As noted above, additional temperature sensors such as thermistors,resistance temperature detectors, pyrometers, infrared temperaturesensors, and other thermometers and temperature sensors can be used fortemperature sensors 410A, 410B, 410C. Each such sensor could beimplemented in the electrodes in a similar manner. For example, thetemperature sensors 410A, 410B, and 410C can be implemented asresistance temperature sensors. A small current (e.g., 5 mA) can bepassed through the resistance temperature sensors. As the temperaturearound the resistance temperature sensors changes, the resistance of theresistance temperature sensors changes in a linear manner. As such, bymeasuring the voltage change across the resistance temperature sensor,the processing circuit 420 can determine a temperature falloff signalfor further analysis by the electrode falloff detector.

Capacitance-Based Falloff Event Detection

In another example, as illustrated in FIGS. 7-15, a capacitance-basedfalloff detection scheme can be included in a wearable medical device.In such a scheme, one or more capacitance or touch sensors can beintegrated into the electrodes. A processing circuit can also beimplemented into the electrodes such that output from the capacitancesensors can be processed (e.g., conditioned and filtered) prior to themedical device controller receiving the capacitance sensor outputs forfurther processing. In such an example, the medical device controllercan monitor the capacitance sensor outputs for all the electrodessimultaneously (or, depending upon processing capabilities,substantially simultaneously) to detect an electrode falloff, e.g., whensome or all of the conductive portion of the sensing electrode losescontact with the patient's skin. For example, the capacitance sensorscan be configured to receive an input capacitance form a patient's bodythrough contact between the patient's skin and the capacitance sensormounted in an electrode. The capacitance sensors can continually measurethe patient's capacitance and transmit the information to a processingcircuit, which can then transmit the information to the medical devicecontroller. The medical device controller, or a component of the medicaldevice controller such as the electrode falloff detector, can receive acapacitive falloff signal including the capacitance change information,process the information to determine that the sensor has likely fallenoff, and provide a notification to a patient (e.g., via an alarm or bydisplaying a message on a display device such as touch screen 220 asdescribed above) that the sensor has likely fallen off. The medicaldevice controller can then instruct the patient to check the specificelectrode and verify that the electrode is making proper connection withthe patient's skin. The following discussion of FIGS. 7-15 providesadditional details related to electrode falloff detection usingcapacitance sensors.

In certain implementations, a capacitor-based falloff detection schemecan be implemented as an oscillation-based system configured to measurechanges in frequency induced by skin contact such as a “Theremin”configuration known to those skilled in the art. The oscillationfrequency can be related to a frequency that results from measuring apatient's electrical characteristics including a measured frequency. Incertain implementations, the measured frequency can be between 200 Hzand 20 kHz. By measuring a change in this frequency (as a result of theelectrode losing contact with the patient's skin), a falloff event canbe detected.

FIG. 7 illustrates a lumped circuit model 700 for electric field sensingthat can be integrated into a capacitive electrode for use in detectingelectrode falloff. The general term electric field sensing encompassesseveral different measurements, which correspond to different currentpathways through this diagram. In sensing mode, a low frequency (e.g.,10-100 kHz) voltage signal is applied to the transmit electrode, labeledT in FIG. 7. A displacement current can flow from the transmitter to theother conductors through the effective capacitors shown in circuit model700. In loading mode, the current flowing from the transmitter can bemeasured. The value of C1, and thus the load on the transmitter, canchange with hand position. For example, when the hand, labeled H in FIG.7, moves closer to the transmitter, the loading current increases.

The term capacitive sensing ordinarily refers to a loading modemeasurement. However, the capacitances other than C1 in FIG. 7 canprovide for other measurements. In transmit mode, when the transmitteris coupled strongly to the body—C1 is very large—so the hand isessentially at the potential of the transmitter. As the body approachesthe receive electrode, labeled R in FIG. 7, the value of C2 (and C0—thetwo are not distinct in this mode) increases, and the displacementcurrent received at R increases. In shunt mode, C0, C1, and C2 can be ofthe same order of magnitude. As the hand approaches the transmitter andreceiver, C1 increases and C0 decreases, leading to a drop in receivedcurrent. As such, the displacement current that had been following tothe receiver is shunted by the hand to ground (hence the term shuntmode). A baseline received current can be measured when the hand is atinfinity, and then subtract later readings from this baseline. With Nordinary capacitive sensors (loading mode), N numbers can be collected.These N numbers represent the diagonal of the capacitance matrix for thesystem of electrodes. In shunt mode, the N(N−1) off-diagonal elementscan be measured. Because the capacitance matrix is symmetrical, thereare ideally only ½ N(N−1) distinct values. In practice, measureddeviations from symmetry provide valuable calibration information.”

In one implementation, the transmit and receive electrodes (labeled “T”and “R” in FIG. 7) are integrated into a capacitive electrode such ascapacitive electrode 1200 described below in the discussion of FIG. 12.In some examples, a capacitive touch/proximity sensor circuit, such asthe PCF8883 sensor circuit manufactured by NXP Semiconductors (EindhovenNetherlands) can be used. The steady state capacitance between thesensor plates, traces, and GND can be compensated for by theauto-calibration mechanism built into the PCF8883 chip. Therefore, theprimary condition to consider for design of sensor plates is to maximizethe capacitance between an approaching portion of the patient's body(e.g., a portion of skin on the patient's chest or back) and the sensorplate. Likewise, the capacitance between the patient's skin and any GNDcan be minimized. As shown in FIG. 8, a ground ring can be provided tominimize stray field effects. The sensor plate size is per definition amajor element in the sensitivity as it is defining the size of theparallel plate capacitor and the touching skin. In certainimplementations, round-shaped sensor plates will provide a maximizedsensing area.

A touch sensor such as the PCF8883 can be used react on certain changesin capacitance instead of measuring absolute capacitance. In certainimplementations, provided the capacitive load is in the specified rangeof 10 pF to 40 pF, any capacitance changes occur and are measured at thespeed of the electrode being removed from the body can be detected. Thesteady state capacitance originating from the layout, slowly changingenvironmental conditions, accumulating dirt, and so on, can becompensated for by the auto-calibration mechanism. Thus, the falloffdetection can have a threshold related to at least one of the followingparameters: 1) absolute capacitance level; 2) rate of capacitance change(e.g. change in Farads per second); 3) the amount of time required forthe capacitance to exceed a particular capacitance level (the changemight be instantaneous or averaged). The falloff detection threshold canbe adjustable based on a particular patient characteristic such asweight, skin dryness, or the baseline electrical characteristics of theskin.

In some implementations, the capacitive electrode can include a tantalumpentoxide coating, known to those skilled in the art. This coating canresult in typical capacitance values of 200-900 nF measured at 1 kHz,with an ESR of 2-10K at 10 Hz. Referring to FIGS. 9 and 10, in someimplementations, the electrode can be split into two or more electricalelements (e.g. E_(1A)/E_(1B) and E_(2A)/E_(2B)) housed in the sameelectrode, as shown in FIG. 10 below. The two (or more) split elementsof the electrode can be disposed in an electrically isolating housingmade, for instance, from a plastic such as PET, polycarbonate, oranother similar electrically isolating plastic. The electrode/skincontact can be modeled as a simple capacitance, or, as shown FIG. 9 (andexplained in greater detail in the discussion of FIG. 19 below) as twoor more of a combination of inductors, resistors or capacitors. In FIG.9, by way of example, the electrode/skin interface can be modeled as aresistor and capacitor in series (e.g. R₁C₁, and R₂C₂) in parallel withanother resistor (e.g. R_(f1) or R_(f2)). Referring to FIGS. 10 and11A-F, the capacitance or impedance measurement circuit may be locatedwithin the electrode housing, the cable connecting the electrode to thedefibrillator, or in the defibrillator itself

Locating the impedance measurement circuit at or near the distal end ofthe cable or within the electrode housing can act to lower any effectsof stray impedance in the cable and thus obtain a more accuratecapacitance/impedance measurement. Switch networks (e.g. S1 _(A-C), S2_(A-C) as shown in FIG. 10) can be provided to switch thecapacitance/impedance circuitry out of the ECG sensing circuitry toimprove signal integrity. For example, switches S1 _(C) and S2 _(C) canbe open and switches S1 _(A,B,) S2 _(A,B) can be closed when measuringcapacitance/impedance to electrically disconnect E1 _(A) from E1 _(B)from each other during capacitance/impedance measurement. Switches S1_(C) and S2 _(C) will be closed and switches S1 _(A,B,) S2 _(A,B) areopen when monitoring the ECG to electrically connect E1 _(A) from E1_(B) from each other disconnect from the capacitance/impedancemeasurement circuitry.

Referring to FIGS. 11A-F, the capacitance/impedance measurementcircuitry can take a number of different configurations, known to thoseskilled in the art. For instance, the measurement circuit may employ thebridge method, resonant method, I-V method, RF-I-V method, networkanalysis method, or auto-balancing bridge method.

FIG. 11A illustrates a bridge implementation of a capacitance/impedancemeasurement circuit. When no current is flowing through the detector D,the value of the unknown impedance Zx can be determined based upon aknown relationship of the other bridge elements. For example, as notedin FIG. 11A, Zx=(Z1/Z2)*Z3.

FIG. 11B illustrates a resonant method implementation of acapacitance/impedance measurement circuit. When the circuit is adjustedto resonance by tuning capacitor C, the unknown impedance values Lx andRx are obtained from a test frequency, the value of C and the value ofQ. Q can be measured directly using a voltmeter or another voltagemeasuring device. In addition to the direct connection shown in FIG.11B, series and parallel connections can be used as well.

FIG. 11C illustrates an I-V method implementation of acapacitance/impedance measurement circuit. In this example, the unknownimpedance Zx can be calculated from measured voltage and current values.The current can be calculated using the voltage measurement across aknown resistor R. In practice, a low loss transformer can be used inplace of R to prevent any circuit-wide effects of using a resistor. Thetransformer, however, can limit the low end of an applicable frequencyrange.

FIG. 11D illustrates a network analysis method implementation of acapacitance/impedance measurement circuit. In this example, adirectional coupler or bridge can be used to detect a reflected signaland a network analyzer can be used to supply and measure the reflectedsignal and an incident signal.

FIG. 11E illustrates an RF I-V method implementation of acapacitance/impedance measurement circuit. Both a low impedance circuitand a high impedance circuit are shown in FIG. 11E. While RF I-Vmeasurement is based upon the same principles of the I-V measurement asshown in FIG. 11C, the RF I-V method can include an impedance matchedmeasurement circuit (e.g., at 50Ω) and a precision test port foroperation at higher frequencies.

FIG. 11F illustrates an auto-balancing bridge method implementation of acapacitance/impedance measurement circuit. In this example, the currentIx can balance with the current Ir which flows through the rangeresistor Rr. The potential at the Low point of the circuit can bemaintained at zero volts, providing a virtual ground. The impedance ofDUT can be calculated using the voltage measured at the High terminal(Vx) and across Rr (Vr).

It should be noted that the circuits provided in FIGS. 11A-F areprovided by way of example only, and additional circuit implementationscan be used for a capacitance/impedance measurement circuit.

FIG. 12 illustrates a sample electrode 1200 that uses acapacitance-based falloff detection process. It should be noted that, asshown in FIG. 12, the electrode 1200 can be an electrode having at leastone side configured to be placed against a patient's skin. For example,the electrode 1200 can be either a sensing electrode or a therapyelectrode as described above. The electrode 1200 can be electricallycoupled to a wire 1205 configured to carrying signals such asphysiological signals measured by the electrode 1200 to another devicesuch as connection pod 130 or controller 120 as described above inreference to FIG. 1. For explanatory purposes only, wire 1205 asdescribed herein can be configured to bi-directionally carry signalsbetween the electrode 1200 and a medical device controller.

The electrode 1200 can include, for example, capacitive sensor 1210. Incertain implementations, the capacitive sensor 1210 can be adielectric-sensing capacitive sensor configured to measure dielectricvalues for materials in contact with the sensor. A capacitive sensorsuch as capacitive sensor 1210 can be constructed to have at leastconductive one side that is coated with a conductive material such ascopper, Indium Tin oxide, a conductive ink, and other similar conductivecoatings. A voltage is applied to the conductive side, resulting in auniform electrostatic field. When a conductive object, such a patient'sskin, touches the uncoated side of the electrode, a capacitive interfaceis formed. Because or resistance inherent in the material used to makethe capacitive sensor, each point about the periphery has a differenteffective capacitance. A controller can measure the effectivecapacitances at various points about the periphery of the capacitivesensor to determine the location of contact between the capacitivesensor the patient's skin. Additionally, a value indicative of theamount of surface area of the capacitive sensor in contact with thepatient's skin can be determined as well.

Referring again to FIG. 12, the capacitive sensor 1210 can be operablyconnected to a processing circuit 1220 by connection 1215. In such anexample, connection 1215 can be a copper (or another similarlyconductive material) wire configured to carry an electrical signalgenerated by the capacitive sensor 1210 to the processing circuit 1220.Processing circuit 1220 can then further process the electrical signalgenerated by the capacitive sensor 1210 and transmit the processedsignal through wire 1205 to a medical device controller such ascontroller 120 as described above.

Depending upon the implementation of the electrode 1200, the processingcircuit 1220 can be implemented in various manners. For example, theprocessing circuit 1220 can be a standalone processing device orintegrated circuit configured to receive an analog signal from thecapacitive sensor 1210 and convert the analog signal to a digital signalfor transmission to a controller over the wire 1205. In certainimplementations, the processing circuit 1220 can be implemented as adirect conversion analog-digital converter configured to generate adigital code for a specific voltage range received from the capacitivesensor 1210. The processing circuit 1220 can be configured to transmitthe digital code over wire 1205 to the medical device controller forfurther processing.

In a specific example, a dielectric-sensing capacitive sensor can beused to measure a dielectric value at each electrode. Adielectric-sensing capacitive sensor can use capacitive coupling (i.e.,the transfer of energy between two objects) to detect and measure adielectric value for anything having a different dielectric than air.For example, human skin has a dielectric value of approximately 0.1 S/m(Siemens per meter) at a 100 Hz sampling rate. In the event of a falloffevent, the measured dielectric value at a dielectric-sensing capacitivesensor would drop to zero. By measuring the changes in measureddielectric values output by the capacitive sensor, the medical devicecontroller (e.g., via the electrode falloff detector 324) can monitorfor an electrode falloff event.

Additional types of capacitive sensors can also be used to detectelectrode falloff events. For example, FIG. 13 illustrates a sampleelectrode 1300 that uses a capacitive touch panel for implementing thefalloff detection process. The electrode 1300 can be electricallycoupled to a wire 1305 configured to carrying signals such asphysiological signals measured by the electrode 1300 to another devicesuch as connection pod 130 or controller 120 as described above inreference to FIG. 1. For explanatory purposes only, wire 1305 asdescribed herein can be configured to bi-directionally carry signalsbetween the electrode 1300 and a medical device controller.

The electrode 1300 can include, for example, capacitive touch panel1310. In certain implementations, the capacitive touch panel 1310 can bean electrostatic-based touch panel. The capacitive touch panel 1310 canbe operably connected to a processing circuit 1320 by connection 1315.In such an example, connection 1315 can be a copper (or anothersimilarly conductive material) wire configured to carry an electricalsignal generated by the capacitive touch panel 1310 to the processingcircuit 1320. Processing circuit 1320 can then further process theelectrical signal generated by the capacitive touch panel 1310 andtransmit the processed signal through wire 1305 to a medical devicecontroller such as controller 120 as described above.

Depending upon the implementation of the electrode 1300, the processingcircuit 1320 can be implemented in various manners. For example, theprocessing circuit 1320 can be a standalone processing device configuredto receive an analog signal from the capacitive touch panel 1310 andconvert the analog signal to a digital signal for transmission to acontroller over the wire 1305. In certain implementations, theprocessing circuit 1320 can be implemented as a direct conversionanalog-digital converter configured to generate a digital code for aspecific voltage range received from the capacitive touch panel 1310.The processing circuit 1320 can be configured to transmit the digitalcode over wire 1305 to the medical device controller for furtherprocessing.

As noted above, in a specific example, an electrostatic-based touchpanel can be used to monitor contact between a patient's skin and theelectrode. An electrostatic-base touch panel typically includes aninsulator such as glass coated with a transparent conductor (such asIndium Tin oxide). As human skin touches the surface of the panel, thecontact distorts the panel's electrostatic field, which is measureablein a change in capacitance of the panel. Additionally, by usinglocation-determining technologies such as triangulation using multipleelectrostatic fields, locational information related to the contactpoint on the panel can be determined. Similarly, the electrostatic-basedtouch panel can use multi-touch technology to detect multiple contactpoints at various locations about the panel. By measuring the changes inmeasured capacitance values output by the capacitive touch panel (at oneor more locations on the panel), the medical device controller (e.g.,via the electrode falloff detector 324) can monitor for an electrodefalloff event. Additionally, if the capacitive touch panel is configuredto measure contact at multiple locations, a quantitative analysis of howmuch surface area of the electrode is touching the patient's skin can bedetermined. In such an example, even if a falloff event has notoccurred, a patient can be instructed to reposition the electrode suchthat there is better overall contact against their skin.

FIGS. 14A and 14B illustrate various configurations for an electricfield (e-field) proximity sensor. An e-field sensor measures changes inelectrical field properties to determine object location. Such a sensorcan be used in concert with an existing capacitive electrode such as theECG electrodes as described herein. The e-field sensors can beconfigured to measure changes in the electrical field produced duringnormal operation of the electrode. In such an arrangement, propertiesmeasured during normal contact with the patient's skin can be used tocalibrate the e-field sensor.

An e-field sensor can be configured to output a variable value that isindicative of what portion of the sensor is detecting a change in anelectrical field. As such, this can allow for various configurations ofthe e-field sensor as shown in FIGS. 14A and 14B. For example, as shownin FIG. 14A, an electrode 1400 can include a ring-shaped e-field sensor1405. As noted above, when using a capacitive sensor, changes inmeasured capacitance at various points on the surface of the capacitivesensor can be used to measure position and amount of contact with, forexample, a patient's skin. A similar concept is used with thering-shaped e-field sensor 1405 where contact can be measured about theperiphery of the electrode 1400. The electrode 1400 can have acapacitive surface that is machined such that a channel is definedtherein. In certain implementations, the capacitive surface can be aplastic sheet coated with a conductive material such as Indium Tinoxide. However, it should be noted that this is merely shown by way ofexample and additional materials such as copper, stainless steel,tantalum, and other similar conductive materials can be used. Thee-field sensor 1405 can then be inserted into the channel such that thee-field sensor 1405 is flush with the surface of the electrode 1400 thatis against the patient's skin. Such a design can be used to measureamount of contact with a patient's skin by measuring a percentage of thecircumference of the ring-shaped e-field sensor 1405 is in contact withthe patient's skin.

FIG. 14B illustrates an alternative design for an e-field based falloffsensing scheme. An electrode 1410 can be machined such that acheckerboard or hash-mark shaped inset is provided therein. An e-fieldsensor 1415 can be fitted into the inset of the electrode 1410 such thatthe e-field sensor sits flush with the surface of the electrode 1400that is in contact with a patient's skin. The electrode 1410 can be aplastic sheet coated with a conductive material such as Indium Tinoxide. However, it should be noted that this is merely shown by way ofexample and additional materials such as copper, stainless steel,tantalum, and other similar conductive materials can be used.

Referring to FIG. 15, the electrode falloff detector can be configuredto receive 1505 the measured capacitive falloff signal from, forexample, one or more of the electrodes. In certain implementations, eachelectrode can be operably connected to a node such as connection pod 130as shown in FIG. 1. The node can include processing circuitry configuredto concatenate, multiplex, or otherwise combine the capacitive falloffsignals from each of the electrodes into a single combined capacitivefalloff signal for transmission to the medical device controller. Insuch an example, the electrode falloff device can be configured toreceive 1505 the combined capacitive falloff signal, divide the combinedcapacitive falloff signal into individual components related to theindividual electrodes, and process the individual components todetermine whether one or more falloff events have occurred. Thus, inthis example, the remainder of the process as shown in FIG. 15 can berepeated for each individual electrode that is associated with thecombined capacitive falloff signal.

In some implementations, each electrode can have a direct connection tothe medical device controller. For example, the medical devicecontroller can include one or more external connectors into which one ormore electrodes can be directly connected. In such an example, theelectrode falloff detector can be configured to receive 1505 thecapacitive falloff signals directly from the individual electrodes.Thus, in this example, the entirety of the process as shown in FIG. 15can be repeated for each electrode operably connected to the electrodefalloff detector.

Referring again to FIG. 15, the electrode falloff detector can befurther configured to determine 1510 whether there has been potentialfalloff event at an electrode. For example, the capacitive falloffsignal can indicate a change in a patient's measured capacitance (e.g.,the patient's measured dielectric value) at a particular electrode. Ifthe electrode falloff detector determines 1510 that there has not been apotential falloff (e.g., no indicated change in the capacitive falloffsignal), the electrode falloff detector can receive 1505 an updatedmeasured capacitive falloff signal.

Conversely, if the electrode falloff detector does determine 1510 thatthere has been a potential falloff event (e.g., there has been ameasured change in a patient's dielectric value), the electrode falloffdetector can further determine 1515 whether a falloff event has likelyoccurred. The electrode falloff detector can be further configured todetermine 1515 whether the measured capacitance change was within anallowed threshold. Based upon various measured patient parameters and/oroperating parameters of the medical device controller and the electrodefalloff detector, a certain threshold of capacitance change can bedetermined as acceptable. In certain implementations, the medical devicecontroller can include a baseline capacitance value or measurement forthe patient. For example, the medical device controller can include abaseline dielectric value of 0.1 S/m for the patient. The acceptablethreshold can be set as plus or minus a certain number of degrees fromthe baseline dielectric value. For example, the acceptable threshold canbe set as plus or minus 0.01 S/m for a particular patient. If theelectrode falloff detector determines 1515 that a dielectric valuechange at an electrode falls within the acceptable threshold, theelectrode falloff detector can receive 1505 an updated capacitivefalloff signal and repeat the process as shown in FIG. 15. Conversely,if the electrode falloff detector determines 1515 that a falloff eventlikely occurred, the electrode falloff detector can provide 1520 anotification of the likely falloff event.

In certain implementations, the electrode falloff detector can provide1520 an alarm to the patient indicating a potential falloff event. Forexample, the alarm can include a visual alarm, an audio alarm, a tactilealarm, a combination of alarms (e.g., alarms that are in a predefinedsequence or that overlap, such as, first, initiating a tactile alert,second, initiating an audible alert, and third, initiating a visualalert on the display), or another similar alarm configured to provide anindication or notification of the potential falloff event to the patientwearing the medical device. In certain implementations, an alarm manager(e.g., alarm module 326 as described above) can be configured to outputone or more alarms in response to a specific event occurring. Forexample, if a treatable cardiac event is detected, the alarm manager canbe configured to cause a high volume audible alarm to occur. In someexamples, a high volume audible alarm can be about 80 db as measured 1meter from the output device (e.g., a speaker or audio resonator). Inthe event of an electrode falloff detection, the alarm manager can beconfigured to output a lower volume alarm. For example, the alarmmanager can be configured to output an alarm about 6-12 db lower thanthe high volume alarm (e.g., an alarm ranging from 68-74 db). In someexamples, the alarm manager can be configured to output a visual alarm.For example, the alarm manager can flash a message or notification onthe medical device's screen (e.g., touchscreen 220 as described above)or another similar visual output device such as one or more LED outputs.In certain implementations, the alarm manager can be configured toprovide a tactile alarm as a standalone alarm or in combination with oneor more of the audio and visual alarms.

In addition to providing the patient notification of the potentialelectrode falloff, the electrode falloff detector can further provide1520 a notification to a remote server or monitoring service of thepotential falloff. For example, the wearable medical device can beoperably connected to a remote server (e.g., remote server 322 asdescribed above) and can be configured to regularly transmit dataindicative of a patient's cardiac activity as well as any detectedevents that occur while the patient is wearing the medical device. Upondetection of a potential electrode falloff, the electrode falloffdetector can provide 1520 a notification such as a time/date stamp andan associated flag indicative of the potential falloff event. Uponreview of the patient's information (e.g., by a technician or apatient's physician) collected by the remote server, the potentialfalloff event can be reviewed as well. In certain implementations, ahigh amount of falloff events (e.g., more than 5 every 2 hours) can beindicative that the patient needs to have their wearable medical deviceadjusted or replaced.

In an example of the process as shown in FIG. 15, the electrode falloffdetector can determine that, over a series of signals representing 30seconds in time, a patient's dielectric value at a specific electrodehas dropped from 0.11 S/m to 0.09 S/m. As this drop falls within a 0.015S/m threshold of a baseline value (e.g., a baseline value of 0.1 S/m),the electrode falloff detector can determine that there has not been afalloff event. To provide higher accuracy of whether a falloff event hasoccurred, the electrode falloff detector can compare measurements frommultiple electrodes. For example, if a dielectric value change (e.g.,0.01 S/m) is consistently measured across multiple electrodes, theelectrode falloff detector can label the dielectric change as a changein patient's body conditions rather than a falloff event. Such labelscan be recorded in memory as flags, dielectric events, or other similarevents for review later by, for example, a technician or other person(such as a physician) reviewing patient-related information recorded bythe wearable medical device.

It should be noted that the thresholds (e.g., for dielectricmeasurements) used for capacitance-based falloff detection can bedetermined individually for each patient using the wearable medicaldevice. For example, a patient can participate in a baselining processincluding measuring the patient's dielectric value during, for example,a garment fitting for the wearable medical device when the patient isfirst subscribed the device. Additionally, the patient can be instructedto perform a physical activity such as a six-minute walk test to measurehow the patient's dielectric value changes during physical activity(e.g., due to sweat or changes in boy temperature). In such an example,the thresholds can be dynamically alterable for a patient depending uponwhether the patient is engaged in physical activity. Additionalinformation such as accelerometer information can also be measuredduring the physical activity to determine what measurable parameters andcharacteristics as associated with the patient during physical activity.

It should be noted that a dielectric-based capacitive sensor and anelectrostatic-based touch panel are shown by way of example only.Additional capacitance sensors such as resistive-based capacitancesensors can be used. For example, a resistive-based capacitive sensorincludes a dynamically changing resistor that has a variable resistancebased upon a capacitance level surrounding the resistor. A small current(e.g., 5 mA) can be passed through the resistive-based capacitivesensor. As the capacitance around the resistive-based capacitive sensorchanges, the resistance changes in a linear manner. As such, bymeasuring the voltage change across the resistive-based capacitivesensor, the processing circuit can determine a capacitive falloff signalfor further analysis by the electrode falloff detector.

Additionally, a Schering Bridge can be used to measure capacitancevalues at an electrode. Any changes in the measured values can beindicative of a falloff event. A Schering Bridge is an AC bridge circuitthat produces a balanced capacitive measurement that is independent offrequency. As such, if frequency changes at the electrode-skin interfaceas a result of bio-electrical changes in the patient's body, an increaseof perspiration between the electrode and the patient's skin, or othersimilar events that can cause a change in measured frequency, the outputof the Schering Bridge remains balanced. However, if a falloff eventoccurs, the two halves of the Schering Bridge will become unbalanced (asa result of the increase in impedance resulting from the falloff event),thereby producing an output that can be indicative of the falloff event.

In certain implementations, frequency modulation can be used to measurecapacitance changes at an electrode. By using known signal phase andamplitude, a received signal can be divided into various segments havingdifferent frequencies. One or more frequencies of interest can then beanalyzed to reduce or eliminate noise that can impact measuring anddetecting a falloff event. In other examples, synchronous demodulationcan be implemented into a falloff detection scheme. In certainimplementations, synchronous demodulation uses a diode rectifier toeliminate sideband information from a signal, thereby resulting in alow-noise carrier band signal for further analysis.

Optical-Based Falloff Event Detection

In another example, as illustrated in FIGS. 16-18, an optical-basedfalloff detection scheme can be included in a wearable medical device.In such a scheme, one or more optical sensors can be integrated into theelectrodes. A processing circuit can also be implemented into theelectrodes such that output from the optical sensors can be processed(e.g., conditioned and filtered) prior to the medical device controllerreceiving the temperature sensor outputs for further processing. In suchan example, the medical device controller can monitor the optical sensoroutputs for all the electrodes simultaneously (or, depending uponprocessing capabilities, substantially simultaneously) to detect anelectrode falloff e.g., when some or all of the conductive portion ofthe sensing electrode loses contact with the patient's skin. Forexample, the optical sensors can be configured to both emit an opticalsignal and receive a reflected signal from the patient's skin. Byanalyzing properties of the received signal, and comparing the receivedsignal to the transmitted signal, a processing circuit can determine adistance measurement between the optical sensor and the patient's skin.The medical device controller, or a component of the medical devicecontroller such as the electrode falloff detector, can receive anoptical falloff signal including distance measurement and changeinformation, process the information to determine whether the sensor haslikely fallen off, and provide a notification to a patient (e.g., via analarm or by displaying a message on a display device such as touchscreen 220 as described above) that the sensor has likely fallen off.The medical device controller can then instruct the patient to check thespecific electrode and verify that the electrode is making properconnection with the patient's skin. The following discussion of FIGS.16-18 provides additional details related to electrode falloff detectionusing capacitance sensors.

FIG. 16 illustrates a sample electrode 1600 that uses an optical-basedfalloff detection process. It should be noted that, as shown in FIG. 16,the electrode 1600 can be an electrode having at least one sideconfigured to be placed against a patient's skin. For example, theelectrode 1600 can be either a sensing electrode or a therapy electrodeas described above. The electrode 1600 can be electrically coupled to awire 1605 configured to carrying signals such as physiological signalsmeasured by the electrode 1600 to another device such as connection pod130 or controller 120 as described above in reference to FIG. 1. Forexplanatory purposes only, wire 1605 as described herein can beconfigured to bi-directionally carry signals between the electrode 1600and a medical device controller.

The electrode 1600 can include, for example, an optical sensor 1610. Incertain implementations, the optical sensor 1610 can be a photoelectricsensor configured to both emit electromagnetic radiation as well asreceive electromagnetic radiation (e.g., radiation reflected by asurface such as the patient's skin). In certain embodiments, theelectromagnetic radiation can be visible light. In other examples, theelectromagnetic radiation can be non-visible light such as infraredlight and ultraviolet light. The optical sensor 1610 can be operablyconnected to a processing circuit 1620 by connection 1615. In such anexample, connection 1615 can be a copper (or another similarlyconductive material) wire configured to carry an electrical signalgenerated by the capacitive sensor 1610 to the processing circuit 1620.Processing circuit 1620 can then further process the electrical signalgenerated by the capacitive sensor 1610 and transmit the processedsignal through wire 1605 to a medical device controller such ascontroller 120 as described above.

Depending upon the implementation of the electrode 1600, the processingcircuit 1620 can be implemented in various manners. For example, theprocessing circuit 1620 can be a standalone processing device configuredto receive an analog signal from the capacitive sensor 1610, amplify andconvert the analog signal to a digital signal for transmission to acontroller over the wire 1605. In certain implementations, theprocessing circuit 1620 can be implemented as a direct conversionanalog-digital converter configured to generate a digital code for aspecific voltage range received from the capacitive sensor 1610. Theprocessing circuit 1620 can be configured to transmit the digital codeover wire 1605 to the medical device controller for further processing.

In a specific example, a photoelectric optical sensor can be used tomeasure a distance between an electrode and a patient's skin. FIG. 17illustrates a sample circuit diagram for a photoelectric optical sensor1700 that can be operably connected to, for example, a processingcircuit (e.g., processing circuit 1620 as shown in FIG. 16) viaconnector 1710 (similar to connection 1615 as shown in FIG. 16). Incertain implementations, the photoelectric optical sensor 1700 will bepositioned in contact with, or substantially adjacent to, a patient'sskin 1705. For example, depending upon the design of the electrode andthe placement of the photoelectric optical sensor 1700, a small gapbetween approximately 0.01 mm and 0.1 mm can be between thephotoelectric optical sensor 1700 and the patient's skin 1705. Incertain implementations, the photoelectric optical sensor 1700 can beslightly inset into the electrode, thereby having a distance ofapproximately 0.05 mm when the electrode is in contact with thepatient's skin 1705.

Referring again to FIG. 17, the photoelectric sensor 1700 can include alight transmitting component 1715. In certain implementations, the lighttransmitting component 1715 can be a light emitting diode configured toemit a focused beam of light in a particular direction (indicated, forexample, by the arrows pointing toward the patient's skin in FIG. 17).The photoelectric sensor 1700 can also include a light receivingcomponent 1720. In certain implementations, the light receivingcomponent 1720 can include a photodiode configured to receive reflectedlight that has bounced or otherwise been reflected back toward to thelight receiving component 1720 (indicated, for example, by the arrowspointing away from the patient's skin in FIG. 17). In certainimplementations, the photoelectric sensor 1700 can further include afilter 1725. For example, the filter 1725 can include a bandpass filterconfigured to filter the reflected light received by the light receivingcomponent 1720 such that only light at frequencies corresponding tolight reflected off skin are transmitted to, for example, processingcircuit 1620 as shown in FIG. 16.

Referring to FIG. 18, the electrode falloff detector can be configuredto receive 1805 the measured optical falloff signal from, for example,one or more of the electrodes. In certain implementations, eachelectrode can be operably connected to a node such as connection pod 130as shown in FIG. 1. The node can include processing circuitry configuredto concatenate, multiplex, or otherwise combine the optical falloffsignals from each of the electrodes into a single combined opticalfalloff signal for transmission to the medical device controller. Insuch an example, the electrode falloff device can be configured toreceive 1805 the combined optical falloff signal, divide the combinedoptical falloff signal into individual components related to theindividual electrodes, and process the individual components todetermine whether one or more falloff events have occurred. Thus, inthis example, the remainder of the process as shown in FIG. 18 can berepeated for each individual electrode that is associated with thecombined optical falloff signal.

In some implementations, each electrode can have a direct connection tothe medical device controller. For example, the medical devicecontroller can include one or more external connectors into which one ormore electrodes can be directly connected. In such an example, theelectrode falloff detector can be configured to receive 1805 the opticalfalloff signals directly from the individual electrodes. Thus, in thisexample, the entirety of the process as shown in FIG. 18 can be repeatedfor each electrode operably connected to the electrode falloff detector.

Referring again to FIG. 18, the electrode falloff detector can befurther configured to determine 1810 whether there has been any changein measured distance between an optical sensor and the patient's skin atan electrode. For example, the optical falloff signal can indicate anupdated distance measurement for the measured distance between thepatient's skin and the optical sensor. If the electrode falloff detectordetermines 1810 that there has not been a change in the measureddistance (e.g., no indicated change in the optical falloff signal), theelectrode falloff detector can receive 1805 an updated measured opticalfalloff signal.

Conversely, if the electrode falloff detector does determine 1810 thatthere has been a change in the measured distance in the optical falloffsignal, the electrode falloff detector can further determine 1815whether the measured distance is outside of an accepted threshold. Basedupon a position of the optical sensor relative to a skin-contactingsurface of the electrode, a certain threshold of measured distance canbe determined as acceptable. In certain implementations, the medicaldevice controller can include a baseline acceptable distance measurementthe patient. For example, the medical device controller can include anacceptable threshold of less than 0.5 mm. If the electrode falloffdetector determines 1815 that a measured distance between the opticalsensor and a patient's skin at an electrode falls within the acceptablethreshold, the electrode falloff detector can determine that no falloffevent has occurred and can receive 1805 an updated optical falloffsignal, thereby repeating the process as shown in FIG. 18. Conversely,if the electrode falloff detector determines 1815 that the measureddistance between the optical sensor and a patient's skin is outside ofthe accepted threshold, the electrode falloff detector can provide 1820a notification of the likely falloff event.

In certain implementations, the electrode falloff detector can provide1820 an alarm to the patient indicating a potential falloff event. Forexample, the alarm can include a visual alarm, an audio alarm, a tactilealarm, a combination of alarms (e.g., alarms that are in a predefinedsequence or that overlap, such as, first, initiating a tactile alert,second, initiating an audible alert, and third, initiating a visualalert on the display), or another similar alarm configured to provide anindication or notification of the potential falloff event to the patientwearing the medical device. In certain implementations, an alarm manager(e.g., alarm module 326 as described above) can be configured to outputone or more alarms in response to a specific event occurring. Forexample, if a treatable cardiac event is detected, the alarm manager canbe configured to cause a high volume audible alarm to occur. In someexamples, a high volume audible alarm can be about 80 db as measured 1meter from the output device (e.g., a speaker or audio resonator). Inthe event of an electrode falloff detection, the alarm manager can beconfigured to output a lower volume alarm. For example, the alarmmanager can be configured to output an alarm about 6-12 db lower thanthe high volume alarm (e.g., an alarm ranging from 68-74 db). In someexamples, the alarm manager can be configured to output a visual alarm.For example, the alarm manager can flash a message or notification onthe medical device's screen (e.g., touchscreen 220 as described above)or another similar visual output device such as one or more LED outputs.In certain implementations, the alarm manager can be configured toprovide a tactile alarm as a standalone alarm or in combination with oneor more of the audio and visual alarms.

In addition to providing the patient notification of the potentialelectrode falloff, the electrode falloff detector can further provide1820 a notification to a remote server or monitoring service of thepotential falloff. For example, the wearable medical device can beoperably connected to a remote server (e.g., remote server 322 asdescribed above) and can be configured to regularly transmit dataindicative of a patient's cardiac activity as well as any detectedevents that occur while the patient is wearing the medical device. Upondetection of a potential electrode falloff, the electrode falloffdetector can provide 1820 a notification such as a time/date stamp andan associated flag indicative of the potential falloff event. Uponreview of the patient's information (e.g., by a technician or apatient's physician) collected by the remote server, the potentialfalloff event can be reviewed as well. In certain implementations, ahigh amount of falloff events (e.g., more than 5 every 2 hours) can beindicative that the patient needs to have their wearable medical deviceadjusted or replaced.

In an example of the process as shown in FIG. 18, the electrode falloffdetector can determine that, for a specific optical falloff signal, aparticular electrode is approximate 0.1 mm from the patient's skin. Asthis measurement is within an acceptable threshold of 0.5 mm, theelectrode falloff detector can determine that no falloff event hasoccurred. If, for an updated optical falloff signal, the measureddistance between the particular electrode and the patient's skin is 1.2mm, the electrode falloff detector can determine and provide anindication that a falloff event has likely occurred.

It should be noted that a photoelectric-based optical sensor asdescribed above is shown by way of example only. Additional opticalsensors can be incorporated for use in detecting a falloff event usingoptical sensors. For example, an infrared-based proximity sensor can beincorporated into an electrode. Similar to the photoelectric sensor, aninfrared proximity sensor transmits an infrared signal that is reflectedby an optical and detected by an infrared detector. Based upon timingand positioning information related to the received signal, distanceinformation between the infrared sensor and the target object can bedetermined.

In certain implementations, a pulse oximetry sensor can be used as anoptical sensor for detecting a falloff event. In such an example, inaddition to merely monitoring distance information, the pulse oximetrysensor can measure additional information related to the patient such aspulse rates and blood oxygen levels. Such information can be used by themedical device controller to determine other information about thepatient such as whether the patient is conscious.

As discussed above, the optical sensors are described as measuringdistance between two objects (e.g., between the optical sensor and apatient's skin). However, in certain implementations, an optical sensorcan return a zero or undefined value for the distance measurement if,for example, the distance between the optical sensor and the patient'sskin exceeds the optical sensor's nominal range, or the maximum distancethe optical sensor can measure. In such an example, the electrodefalloff detector can be programmed to immediately respond to a zero orundefined measurement as indicating a falloff event.

In additional implementations, alternative optical sensors can beimplemented into an electrode. For example, a camera-based opticalsensor or sensor assembly can be implemented. One or more light sourcescan be integrated into the optical sensor assembly at, for example, thecenter of an electrode. One or more cameras (e.g., arranged in a ringabout the periphery of the electrode) can be configured to measurereflected light produced as a result of the one or more light sourcesreflecting off a surface such as the patient's skin. A high value formeasured light reflection can be indicative of a space between the lightsource and the patient's skin, which can be interpreted as a falloffevent. In alternative designs, multiple light sources can be positionedabout the periphery of the electrode with one or more cameras or otherlight detectors positioned at or about the middle of the electrode. Thelight sources can each be configured to output a certain color orfrequency of light such that each light source is identifiable. The oneor more cameras can be configured to measure reflected light from thelight sources. By analyzing the received light signals for itsindividual components, a processor can determine which portions of theelectrode have lost contact with the patient's skin (e.g., identifying apartial falloff event that can be indicative of a poorly fittinggarment, improper electrode placement, and other similar factors thatcan cause a falloff event).

Impedance-Based Falloff Detection

In another example, as illustrated in FIGS. 19 and 20, animpedance-based falloff detection scheme can be included in a wearablemedical device. In such a scheme, the existing capacitive measuringproperties of a sensing and/or therapy electrode can be used to measurechanges in impedance and/or capacitance between an electrode-skininterface, e.g., the area of contact between the sensing and/or therapyelectrode and a patient's skin. In such an example, the medical devicecontroller can monitor the outputs of each sensing electrode todetermine changes in impedance that could be indicative of a falloffevent, e.g., when some or all of the conductive portion of the electrodeloses contact with the patient's skin.

The medical device controller, or a component of the medical devicecontroller such as the electrode falloff detector, can receive animpedance falloff signal including a measurement of the currentimpedance at the sensor-skin interface for each electrode. Based uponthe size of the electrode, and the total area of the electrode that isconfigured to contact the patient's skin, the electrode can be animpedance sensing electrode configured to measure a range of impedances.For example, the electrodes can be configured to measure between50-200Ω, 200-400Ω, 400Ω-10 kΩ, 10 kΩ-1 MΩ, 1 MΩ-10 MΩ, 10 MΩ to 100 MΩ,100 MΩ to 1 GΩ, and 1 GΩ to 10 GΩ. In certain implementations, a totalimpedance of the sensor-skin interface can be between 400Ω and 1 kΩ.

FIG. 19 illustrates a sample electrode-skin interface 1900 as well as anequivalent circuit model 1910. It should be noted that theelectrode-skin interface 1900 as shown in FIG. 19 is directed to a dryelectrode 1902 placed in direct contact with the patient's skin andconfigured to measure physiological signals such as ECG signals. Forexample, the electrode 1902 can be a capacitive-based sensing electrodeincluding a tantalum conductive portion configured to be placed directlyproximate to the patient's skin. In certain implementations, theelectrode 1902 can include one or more isolated sensing areas or sensors1903 disposed about the surface of the electrode 1902 and configured tomeasure an impedance value between the electrode 1902 and the patient'sskin. The electrode-skin interface 1900 can include an electrolyte layer1904. The electrolyte layer 1904 can include any liquids such as sweatthat might be present on the patient's skin, as well as sweat and otherliquids contained within the patient's skin. The electrode-skininterface 1900 can also include the epidermis 1906 positioned below theelectrolyte layer 1904. The epidermis 1906 is the outermost layer of thepatient's skin. It should be noted that the electrolyte payer 1904 andthe epidermis 1906 can be physically present in the same layer of thepatient's skin. However, for modeling purposes when measuring theimpedance of an electrode-skin interface, it can be advantageous tomodel the two layers separately as they each have distinct electricalcharacteristics.

The epidermis layer 1906 provides a barrier for the human body againstinfection from various pathogens, as well as regulates the amount ofwater released by the body. The epidermis can vary in thickness, rangingfrom about 0.5 mm to about 1.5 mm depending upon what part of the bodyis being measured. The internal resistance of the epidermis can vary inaccordance with the thickness of the epidermis.

As shown in FIG. 19, below the epidermis layer 1906 is the dermis layer1908. The dermis is a layer of skin positioned between the epidermis andvarious subcutaneous tissues. The dermis includes various componentssuch as hair follicles, sweat glands, lymphatic vessels and bloodvessels. The dermis can also vary in thickness, ranging from about 0.6mm to about 3.0 mm. Similar to the epidermis, the internal resistance ofthe dermis can vary in accordance with the thickness of the dermis.

In the sample electrode-skin interface 1900 as shown in FIG. 19, eachspecific layer of the interface can include unique and distinctelectrical properties. For example, the epidermis layer 1906 can have anaverage impedance of approximately 1 kΩ. The electrolyte layer 1904, asa result of including a high number of conductive materials such as saltwater, can have a low impedance as compared to the epidermis layer. Forexample, the electrolyte layer can have an impedance of less than 1Ω.Similarly, due to the presence of various sweat glands, blood vesselsand other similar components, the dermis layer can have a lowerimpedance as compared to the epidermis layer. For example, the dermislayer can have an average impedance of approximately 500Ω. As such, thevarious layers of skin (i.e., the electrolyte layer 1904, the epidermis1906, and the dermis 1908) can have an average combined impedance ofabout 1.5 kΩ. In certain implementations, depending upon various aspectsspecific to the patient wearing the sensing electrode (e.g., % body fat,amount of muscle, body temperature), the average combined impedance foreach patient can vary. As such, the average combined impedance for apatient can range from approximately 1 kΩ to 100 kΩ. However, it shouldbe noted that the above resistance and impedance numbers are provided byway of example only. Actual patient impedances can vary outside of theprovided ranges as described above, and can be measured more accuratelyon a patient-by-patient basis.

When using an impedance-based falloff detection scheme, baseline valuesfor various components in an electrical model can be determined andstored. Then, by using a set voltage and measured current at the sensingelectrodes, the impedance at the electrode-skin interface can bemodeled. Based upon a modeled value, a processing device, such as amedical device controller as described above, can determine a modeledimpedance level at the sensing electrode and, based upon the determinedimpedance level, determine the likelihood that a falloff event hasoccurred. Electrical circuit model 1910 as shown in FIG. 19 can be usedas a model for the various electrical properties of theelectrode-patient interface 1900. In the circuit model 1910, U_(eq) canrepresent the electrode potential, e.g., the voltage at the electrode.In certain implementations, the electrode potential can range from 2-12v. For example, the electrode potential of the electrode can be 2.5 v.In the circuit model 1910, resistor R_(ct) can represent the chargetransfer resistance at the electrode-skin interface. For example, R_(ct)provides a measurement of the actual resistance between the electrodeand the patient's skin. By measuring changes in this value, the medicaldevice controller can determine whether a falloff event has occurred,e.g., whether the electrode has lost contact with the patient's skin.The circuit model 1910 can also include capacitor C_(DC) in parallelwith resistor R_(ct). Capacitor C_(DC) can represent the capacitance ofthe sensing electrode. For example, capacitor C_(DC) can be configuredto be approximately 0.5-2 μF. In certain implementations, capacitorC_(DC) can be configured to be approximately 1.0 μF.

The circuit model 1910 can also include a resistor R_(L) and a capacitorC_(T) positioned in parallel to each other, and the combination of theresistor R_(L) and the capacitor C_(T) can be positioned in series withthe combination of resistor R_(ct) and capacitor C_(DC). Resistor R_(L)can represent the resistance of the electrolyte layer (e.g., electrolytelayer 1904 as described above). As noted above, the electrolyte layercan have a resistance of approximately 1Ω. As such, resistor R_(L) inthe circuit model 1910 can have a resistance of approximately 1Ω.Capacitor C_(T) can be configured to represent the capacitive behaviorof the electrode-skin interface as a result of a lack of conductivematerial such as conductive gel. The capacitor C_(T) can be configuredto be about 0.1-0.5 pF. In certain implementations, the capacitor C_(T)can be set to approximately 0.25 pF in the circuit model 1910.

The circuit model can also include a potential U_(S) that represents thesurface energy potential of the patient's skin. The energy potentialU_(S) can be positioned in the circuit model in series following thecombination of resistor R_(L) and the capacitor C_(T). As noted above,electrical properties of a human's skin can vary greatly betweenindividual bodies, but generally a human's potential energy at theirskin (e.g., at the top of the dermis layer) is approximately 5-25 mV.Thus, for example, the energy potential U_(S) can be set toapproximately 10 mV in the circuit model 1910.

The circuit model 1910 can also include a resistor R_(S) and a capacitorC_(S) positioned in parallel to each other, and the combination of theresistor R_(L) and the capacitor C_(T) positioned in series with theenergy potential U_(S). Resistor R_(S) can represent the resistance ofthe epidermis layer (e.g., epidermis layer 1906 as described above). Asnoted above, the epidermis layer can have a resistance of approximately1 kΩ. As such, resistor R_(S) in the circuit model 1910 can have aresistance of approximately 1 kΩ. Capacitor C_(S) can be configured torepresent the capacitive behavior of the dermis layer. The capacitorC_(S) can be configured to be about 1-1.5 pF. In certainimplementations, the capacitor C_(S) can be set to approximately 1.25 pFin the circuit model 1910.

The model circuit 1910 can also include a resistor R_(SUB) thatrepresents the resistance of the dermis layer (e.g., dermis layer 1908).As shown in FIG. 19, the resistor R_(SUB) can be positioned in seriesand following the combination of resistor R_(S) and a capacitor C_(S).As noted above, the dermis can have a resistance of approximately 500Ω.As such, resistor R_(SUB) can have a resistance of approximately 500Ω.

As such, the circuit represented in circuit model 1910, has a combinedresistance of approximately 1500Ω plus the impedance represented byresistor R_(CT). Thus, for a set current being delivered to the skin bythe electrode, a constant value can be modeled for the resistance acrossR_(CT). For example, using a 1.0 mA current, an impedance ofapproximately 1 kΩ can be modeled for resistor R_(CT). Thus, bymonitoring any changes in the current provided by the sensing electrodeat the electrode-skin interface, changes in the impedance between thesensing electrode and the patient's skin can be identified and, basedupon the impedance changes, a falloff event can be determined.

It should be noted that the values used in the above description areprovided by way of example only. For example, the sample values forcurrent and voltage provided by the electrode are by way of example onlyand can be altered based upon various criteria such as type of sensingelectrode being used, patient information such as % body fat, as well asbaseline information from previous periods when the patient was beingmonitored.

FIG. 20 depicts a sample process flow for detecting an electrode falloffevent using an impedance-based detection scheme. For example, anelectrode falloff detector (e.g., electrode falloff detector 324 asshown in FIG. 3 and described above) can be configured to receive 2005an impedance falloff signal from, for example, one or more of theelectrodes. In certain implementations, the impedance falloff signal caninclude one or more modeled impedance measurements for resistor R_(ct)as modeled using circuit model 2010.

In certain implementations, each electrode can be operably connected toa node such as connection pod 130 as shown in FIG. 1. The node caninclude processing circuitry configured to concatenate, multiplex, orotherwise combine the impedance falloff signals from each of theelectrodes into a single combined optical falloff signal fortransmission to the medical device controller. In such an example, theelectrode falloff device can be configured to receive 2005 the combinedimpedance falloff signal, divide the combined impedance falloff signalinto individual components related to the individual electrodes, andprocess the individual components to determine whether one or morefalloff events have occurred. Thus, in this example, the remainder ofthe process as shown in FIG. 20 can be repeated for each individualelectrode that is associated with the combined impedance falloff signal.

In some implementations, each electrode can have a direct connection tothe medical device controller. For example, the medical devicecontroller can include one or more external connectors into which one ormore electrodes can be directly connected. In such an example, theelectrode falloff detector can be configured to receive 2005 theimpedance falloff signals directly from the individual electrodes. Thus,in this example, the entirety of the process as shown in FIG. 20 can berepeated for each electrode operably connected to the electrode falloffdetector.

Referring again to FIG. 20, the electrode falloff detector can befurther configured to determine 2010 whether there has been any changein a modeled impedance value at an electrode-skin interface. Forexample, the impedance falloff signal can indicate an updated modeledimpedance indicating that the impedance between the electrode and thepatient's skin has either increased or decreased. If the electrodefalloff detector determines 2010 that there has not been a change in themodeled impedance (e.g., no indicated change in the impedance falloffsignal), the electrode falloff detector can receive 2005 an updatedimpedance falloff signal.

Conversely, if the electrode falloff detector does determine 2010 thatthere has been a change in the modeled impedance in the impedancefalloff signal, the electrode falloff detector can further determine2015 whether the modeled impedance is outside of an accepted threshold.In certain implementations, the medical device controller can include abaseline acceptable impedance. For example, the medical devicecontroller can include an acceptable threshold of between 500Ω and 2.5kΩ. At room temperature, the resistivity of air is approximately2×10¹⁶Ω/m. As such, during a falloff event, the modeled impedance can beexpected to increase greatly (e.g., to more than 10 MΩ) as there is aportion of air between the electrode and the patient's skin increasingthe impedance. As such, the threshold of acceptable modeled impedancescan be varied based upon the patient's body type while still beingmultiple orders of magnitude away from the impedance of air.

Additionally, the electrode falloff detector can also determine 2015whether there has been a partial falloff of an electrode. As the modeledimpedance is directly related to the amount of the electrode surfacethat is contact with the patient's skin, variations in the modeledimpedance can indicate that there has been a partial falloff event(e.g., when only a portion of the sensing electrode has lost contactwith the patient's skin). For example, a modeled impedance of 2.5 kΩ canindicate that approximately 50% of the electrode has lost contact withthe patient's skin. Other modeled impedances can indicate a differentpercentage of an electrode has lost contact with the patient's skin. Forexample, various modeled impedances can indicate that approximately20%-50% of the electrode has lost contact with the patient's skin,approximately 25%-75% of the electrode has lost contact with thepatient's skin, and approximately 50%-80% of the electrode has lostcontact with the patient's skin. In other implementations, the electrodefalloff detector can be programed to only determine whether there iscontact between the electrode and the patient's skin or not. In such animplementation, the electrode falloff detector can be programed toprovide a simple yes/no response (e.g., yes the electrode and skin arein contact or no they are not).

If the electrode falloff detector determines 2015 that the modeledimpedance between the electrode and a patient's skin at an electrodefalls within the acceptable threshold, the electrode falloff detectorcan determine that no falloff event has occurred and can receive 2005 anupdated impedance falloff signal, thereby repeating the process as shownin FIG. 20. Conversely, if the electrode falloff detector determines2015 that the modeled impedance between the electrode and a patient'sskin is outside of the accepted threshold, the electrode falloffdetector can provide 2020 a notification of the likely falloff event or,as noted above, warn about a potential partial falloff event.

In certain implementations, the electrode falloff detector can provide2020 an alarm to the patient indicating a potential falloff event. Forexample, the alarm can include a visual alarm, an audio alarm, a tactilealarm, a combination of alarms (e.g., alarms that are in a predefinedsequence or that overlap, such as, first, initiating a tactile alert,second, initiating an audible alert, and third, initiating a visualalert on the display), or another similar alarm configured to provide anindication or notification of the potential falloff event to the patientwearing the medical device. In certain implementations, an alarm manager(e.g., alarm module 326 as described above) can be configured to outputone or more alarms in response to a specific event occurring. Forexample, if a treatable cardiac event is detected, the alarm manager canbe configured to cause a high volume audible alarm to occur. In someexamples, a high volume audible alarm can be about 80 db as measured 1meter from the output device (e.g., a speaker or audio resonator). Inthe event of an electrode falloff detection, the alarm manager can beconfigured to output a lower volume alarm. For example, the alarmmanager can be configured to output an alarm about 6-12 db lower thanthe high volume alarm (e.g., an alarm ranging from 68-74 db). In someexamples, the alarm manager can be configured to output a visual alarm.For example, the alarm manager can flash a message or notification onthe medical device's screen (e.g., touchscreen 220 as described above)or another similar visual output device such as one or more LED outputs.In certain implementations, the alarm manager can be configured toprovide a tactile alarm as a standalone alarm or in combination with oneor more of the audio and visual alarms.

In addition to providing the patient notification of the potentialelectrode falloff, the electrode falloff detector can further provide2020 a notification to a remote server or monitoring service of thepotential falloff. For example, the wearable medical device can beoperably connected to a remote server (e.g., remote server 322 asdescribed above) and can be configured to regularly transmit dataindicative of a patient's cardiac activity as well as any detectedevents that occur while the patient is wearing the medical device. Upondetection of a potential electrode falloff, the electrode falloffdetector can provide 2020 a notification such as a time/date stamp andan associated flag indicative of the potential falloff event. Uponreview of the patient's information (e.g., by a technician or apatient's physician) collected by the remote server, the potentialfalloff event can be reviewed as well. In certain implementations, ahigh amount of falloff events (e.g., more than 5 every 2 hours) can beindicative that the patient needs to have their wearable medical deviceadjusted or replaced.

In the above discussion, a circuit model for modeling impedance has beendescribed. However, this circuit model is provided by way of exampleonly. In additional implementations, a circuit model can be created formodeling capacitance (e.g., the value of capacitor C_(DC) as included incircuit model 1910). In such an example, the values for voltage, currentand resistance can be fed into the model to model the capacitance at theelectrode-skin interface. Additionally, various frequencies can beapplied to the model. By using a known frequency, and varying thefrequency over a range of frequencies, the capacitance of theelectrode-skin interface can be modeled across the range of frequencies.

Combined Falloff Detection

The falloff detection schemes and processes as described above can beimplemented as standalone options for falloff detection as well as partof a combined falloff detection scheme. For example, as noted above, ifa patient is in a warm environment (e.g., where the ambient temperatureis close to human body temperature), the temperature-based falloffdetection process as described above may not immediately recognize allfalloff events. Additionally, in certain implementations, theoptical-based sensing can return a false indication of a falloff event.For example, if the patient's body is curved or bent such that only theportion of the electrode containing the optical sensor is away from theskin, but is still making good enough contact to provide a strongsensing signal (from a sensing electrode) or to provide a therapeuticshock (for a therapy electrode), the electrode falloff detector mayregister the event as a falloff event when the sensor is actually stillproperly positioned. By including the impedance falloff detection schemeas describe above in addition to the optical sensor, the electrodefalloff detector can then determine that at least a portion of theelectrode is still in contact with the patient's skin.

As such, a combination of the above discussed falloff detectiontechniques can be implemented in a wearable medical device. For example,both temperature-based detection and capacitance-based detection sensorscan be integrated into a single electrode. In such implementations, theelectrode falloff detector can be configured to monitor both temperaturechanges as well as capacitance changes at each electrode. Similarly,optical-based falloff detection can be implement with one or both oftemperature-based falloff detection and capacitance-based falloffdetection.

Additionally, the acceptable thresholds for determining a falloff eventcan be configured, programmed, or otherwise altered based upon the typeof electrodes being used and their associated operating parameters, aswell as the combinations of detection schemes being used.

Although the subject matter contained herein has been described indetail for the purpose of illustration, it is to be understood that suchdetail is solely for that purpose and that the present disclosure is notlimited to the disclosed embodiments, but, on the contrary, is intendedto cover modifications and equivalent arrangements that are within thespirit and scope of the appended claims. For example, it is to beunderstood that the present disclosure contemplates that, to the extentpossible, one or more features of any embodiment can be combined withone or more features of any other embodiment.

Other examples are within the scope and spirit of the description andclaims. Additionally, certain functions described above can beimplemented using software, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions can alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations.

What is claimed:
 1. A system for detecting contact between an electrodeand a patient's skin, the system comprising: an electrode assemblycomprising at least one electrode configured to be disposedsubstantially proximate to the patient's skin and configured to at leastone of sense an ECG signal of the patient and provide one or moretherapeutic pulses to the patient; one or more sensors disposed on theelectrode assembly and isolated from the electrode, the one or moresensors configured to measure one or more properties to determinecontact between the electrode and the patient's skin; and a controllerconfigured to receive data representing the measured one or moreproperties and determine, based at least in part on the received data,whether the electrode is in contact with the patient's skin.
 2. Thesystem of claim 1, further comprising an alarm module operablyconfigured to the controller and configured to output at least one alarmif the controller determines that the electrode is not in contact withthe patient's skin.
 3. The system of claim 2, wherein the at least onealarm comprises at least one of an audio alarm, a visual alarm, atactile alarm, and combinations thereof.
 4. The system of claim 1,further comprising a network interface operably connected to thecontroller and configured to establish communication between thecontroller and a remote computing device such that, if the controllerdetermines that the electrode is not in contact with the patient's skin,a notification is sent to the remote computing device indicating anelectrode falloff event.
 5. The system of claim 1, wherein the one ormore sensors are disposed on the electrode.
 6. The system of claim 1,wherein the electrode comprises an electrode configured to sense atleast one surface ECG signal.
 7. The system of claim 6, wherein theelectrode comprises an impedance detection range selected from at leastone of 50Ω-200Ω, 200Ω-400Ω, 400Ω-10 kΩ, 10 kΩ-1 MΩ, and 1 MΩ-10 MΩ. 8.The system of claim 1, wherein the one or more properties define a levelof contact between the electrode and the patient's skin.
 9. The systemof claim 8, wherein the controller is further configured to compare thelevel of contact to a contact threshold level of contact to determine afalloff event.
 10. The system of claim 1, wherein the one or moresensors are configured to measure an impedance level between theelectrode and the patient's skin.
 11. The system of claim 10, whereinthe controller is further configured to model an electrical circuitrepresentative of an interface between the one or more sensing locationsand the patient's skin based at least upon the measured impedance level,and determine whether the electrode is in contact with the patient'sskin.
 12. The system of claim 11, wherein the modeled electrical circuitis configured to simulate an impedance level between the electrodeassembly and the patient's skin, the modeled electrical circuitcomprising at least a first cell configured to simulate a stored energylevel of the electrode, a first capacitive and resistive pair configuredto simulate the electrode, a second capacitive and resistance pairconfigured to simulate an electrolyte layer positioned between theelectrode and the patient's skin, a second cell configured to simulatean energy potential between the electrode and the patient's skin, asecond capacitive and resistance pair configured to simulate anepidermis layer of the patient, and a resistance configured to simulatea dermis layer of the patient.
 13. The system of claim 1, wherein theone or more properties include at least one of temperature, capacitance,measured distance between the electrode and the patient's body, andoxygen saturation of the patient's blood.
 14. The system of claim 1,wherein the one or more sensors comprise a combination of multiplesensor types selected from at least a temperature sensor, a capacitivesensor, and an optical sensor.
 15. The system of claim 14, wherein themultiple sensor types are configured to operate in concert to providemultiple measurements of the one or more properties determined by theposition of the electrode in relation to the patient's body.
 16. Thesystem of claim 15, wherein the controller is further configured toreceive data representing the measured one or more properties from eachof the multiple sensor types to determine whether the electrode is incontact with the patient's body.
 17. A wearable medical system,comprising: an externally wearable cardiac monitoring device; anelectrode configured to be coupled to the externally wearable cardiacmonitoring device and configured to be disposed substantially proximateto the patient's skin to at least one of sense an ECG signal of thepatient and provide one or more therapeutic pulses to the patient; atleast one temperature sensor disposed on the electrode, the at least onetemperature sensor to measure a value indicative of a temperature at aninterface of the electrode and the patient's skin; and a controllerhoused within the externally wearable cardiac monitoring device, thecontroller configured to receive data representing the measured valueand determine, based at least in part on the received data, whether theelectrode is in contact with the patient's skin.
 18. The medical systemof claim 17, wherein the at least one temperature sensor is disposed ona first surface of the electrode positioned substantially proximate tothe patient's skin, the medical system further comprising a secondtemperature sensor disposed on a second surface of the electrode andconfigured to be positioned away from the patient's skin, the secondtemperature sensor configured to measure ambient temperature.
 19. Thesystem of claim 17, wherein the controller is configured to determinewhether the electrode is in contact with the patient's body based on adetermination of whether the measured temperature has changed fasterthan a threshold rate of change.
 20. The system of claim 17, wherein thecontroller is configured to determine whether the electrode is incontact with the patient's body based on a determination of whether themeasured temperature has exceeded, for at least a threshold period oftime, a threshold of temperature change from an expected temperature.21. The system of claim 20, further comprising an ambient temperaturesensor configured to measure ambient temperature, wherein the controlleris configured to receive data representing the ambient temperature. 22.The system of claim 17, further comprising an accelerometer to measuremotion associated with the sensing electrode, wherein the controller isconfigured to receive data representing the measured motion.
 23. Thesystem of claim 17, wherein the at least one temperature sensorcomprises at least one of a thermocouple, a thermistor, a resistancetemperature detector, a pyrometer, and an infrared temperature sensor.24. The system of claim 17, wherein the at least one temperature sensoris thermally insulated from a surface of the electrode.
 25. The systemof claim 24, further comprising an insulating material positionedbetween the at least one temperature sensor and the surface of theelectrode to thermally insulate the at least one temperature sensor. 26.A medical system for detecting contact between an electrode and apatient's skin, the system comprising: an externally wearable cardiacmonitoring system; an electrode configured to be coupled to theexternally wearable cardiac monitoring device and configured to bedisposed substantially proximate to the patient's skin and configured toat least one of sense an ECG signal of the patient and provide one ormore therapeutic pulses to the patient; at least one capacitive sensordisposed on the electrode and configured to be positioned substantiallyproximate the patient's skin to measure a capacitance value between aninterface of the electrode and the patient's skin; and a controllerhoused within the externally wearable cardiac monitoring device, thecontroller configured to receive data representing the measuredcapacitance value and determine, based at least on the received data,whether the electrode is in contact with the patient's skin.
 27. Thesystem of claim 26, wherein the one or more sensing locations configuredto measure capacitance comprise at least one of a dielectric-basedcapacitive sensor, an electrostatic-based touch panel, and aresistive-based capacitance sensor.
 28. A medical system for detectingcontact between an electrode and a patient's skin, the systemcomprising: an externally wearable cardiac monitoring system; anelectrode configured to be coupled to the externally wearable cardiacmonitoring device and configured to be disposed substantially proximateto the patient's skin and configured to at least one of sense an ECGsignal of the patient and provide one or more therapeutic pulses to thepatient; at least one optical sensor disposed on the electrode andconfigured to be positioned substantially proximate the patient's skinand to measure a value indicative of a distance between the electrodeand the patient's skin; and a controller housed within the externallywearable cardiac monitoring device, the controller configured to receivedata representing the measured value indicative of the distance betweenthe electrode and the patient's skin and determine, based at least onthe received data, whether the electrode is in contact with thepatient's skin.
 29. The system of claim 28, wherein the one or moresensing locations configured to optically measure a distance between theelectrode and the patient's body comprise at least one of aphotoelectric sensor, an infrared proximity sensor, and a pulse oximetrysensor.
 30. The system of claim 29, wherein the pulse oximetry sensor isconfigured to measure blood oxygen saturation information for thepatient.